Whenever I try to compile my code in Visual C++ 6.0, I get three errors: driver.obj : error LNK2001: unresolved external symbol "public: __thiscall Rational::Rational(int,int)" (??0Rational@@QAE@HH@Z)
Rational.obj : error LNK2001: unresolved external symbol "public: __thiscall Rational::Rational(int,int)" (??0Rational@@QAE@HH@Z)
Debug/driver.exe : fatal error LNK1120: 1 unresolved externals

My code is:

// driver.cpp : Defines the entry point for the console application.
//
#include "stdafx.h" 
#include <iostream> 
using std::cout; 
using std::endl;
using std::cin; 
#include "Rational.h"


int main()
{
int a1;
int a2;
int b1;
int b2;
   cout<<"Input the numerator of the first number.";
cin>>a1;
cout<<"Input the denominator of the second number.";
cin>>a2;
cout<<"Input the numerator of the second number.";
cin>>b1;
cout<<"Input the denominator of the second number.";
cin>>b2;
   Rational c( a1, a2 ), d( b1, b2 ), x; // creates three rational objects 

   c.printRational(); // prints rational object c
   cout << " + ";
   d.printRational(); // prints rational object d                
   x = c.addition( d ); // adds object c and d; sets the value to x

   cout << " = ";
   x.printRational(); // prints rational object x
   cout << '\n'; //New line
   x.printRational(); // prints rational object x    
   cout << " = ";
   x.printRationalAsDouble(); // prints rational object x as double
   cout << "\n\n"; //Skip a line

   c.printRational(); // prints rational object c
   cout << " - ";
   d.printRational(); // prints rational object d
   x = c.subtraction( d ); // subtracts object c and d 
           
   cout << " = ";
   x.printRational(); // prints rational object x
   cout << '\n';
   x.printRational(); // prints rational object x
   cout << " = ";
   x.printRationalAsDouble(); // prints rational object x as double
   cout << "\n\n";

   c.printRational(); // prints rational object c
   cout << " x ";
   d.printRational(); // prints rational object d
   x = c.multiplication( d ); // multiplies object c and d
                            
   cout << " = ";
   x.printRational(); // prints rational object x
   cout << '\n';
   x.printRational(); // prints rational object x
   cout << " = ";
   x.printRationalAsDouble(); // prints rational object x as double
   cout << "\n\n";

   c.printRational(); // prints rational object c
   cout << " / ";
   d.printRational(); // prints rational object d    
   x = c.division( d ); // divides object c and d
                            
   cout << " = ";
   x.printRational(); // prints rational object x        
   cout << '\n';
   x.printRational(); // prints rational object x
   cout << " = ";
   x.printRationalAsDouble(); // prints rational object x as double
   cout << endl;
   return 0;
} // end main



// Rational.cpp: implementation of the Rational class.
//
//////////////////////////////////////////////////////////////////////


#include "stdafx.h"
#include "Rational.h"
#include <iostream>
using std::cout;
using std::cin;

//////////////////////////////////////////////////////////////////////
// Construction/Destruction
//////////////////////////////////////////////////////////////////////


    Rational Rational::addition(const Rational &m)
{
    Rational t;
int lcd;
int mult1;
int mult2;
lcd=Rational::LCM(denominator, m.denominator);
mult1=denominator/lcd;
mult2=m.denominator/lcd;
t.numerator=(mult1 * denominator) + (mult2 * m.denominator);
t.denominator=lcd;

    return t;
}

Rational Rational::subtraction(const Rational &m)
{
    Rational t;
int lcd;
int mult1;
int mult2;
lcd=Rational::LCM(denominator, m.denominator);
mult1=denominator/lcd;
mult2=m.denominator/lcd;
t.numerator=(mult1 * denominator) - (mult2 * m.denominator);
t.denominator=lcd;
    return t;
}

Rational Rational::multiplication(const Rational &m)
{
    Rational t;
    t.numerator = numerator * m.numerator;
    t.denominator = denominator * m.denominator;
    return t;
}

Rational Rational::division(const Rational &m)
{
    Rational t;
    t.numerator = numerator * m.denominator;
    t.denominator =  denominator * m.numerator;
    return t;
}

void Rational::printRational()
{
cout << numerator<<"/"<<denominator;
}

void Rational::printRationalAsDouble()
{
cout<<numerator/1.0/denominator;
}

int Rational::LCM(int x,int y)
   {
      int prod;
      if(y%x==0)
         return y;
      else
      {
         prod=x*y;
         while(x!=y) // get the GCD of 2 given integers
         {
            if(x>y)
               x=x-y;
            else
               y=y-x;   //x now is the GCD
         }
         return LCM(y,prod/x);  //recurse, changing x to y and vice versa
      }   //LCM = (x*y)/(GCD)
}


// Rational.h: interface for the Rational class.
//
//////////////////////////////////////////////////////////////////////

#ifndef RATIONAL_H
#define RATIONAL_H



class Rational 
{
public:
   Rational( int = 0, int = 1 ); // default constructor
   Rational addition( const Rational &); // function addition
   Rational subtraction( const Rational & ); // function subtraction
   Rational multiplication( const Rational & ); // function multi.
   Rational division( const Rational & ); // function division
   void printRational (); // print rational format
   void printRationalAsDouble(); // print rational as double format
   int LCM(int x, int y);
private:
   int numerator; // integer numerator    
   int denominator; // integer denominator
   void reduction(); // utility function
}; // end class Rational

#endif

Sorry for the long post, I just haven't been able to figure this out :'(

You only defined the constructer, you now need to derive it in the .cpp file of the class and finish it.