Your problem is assuming that x1 and x2 are integers when you print them out.
Additionally, a,b,c are all likely to be floating point numbers. So read them in as floats. So make your output statement printf("%g and %g\n",x1,x2); Also you can use the sqrt function in line k=sqrt(d); .

HOWEVER: Can I just point out that using the formula given is a horrible way to calculate the roots to a quadratic equation due to the numeric instability of the solution. The "better" way is to calculate x1,x2 and determine which biggest root
(either in terms of magnitude or accuracy as a solution) and use the relationship x1*x2=c/a to get the other root.

There are further tricks to getting better roots from a quadratic equation, particularly if the roots are close, if b^2-4ac is near zero ,e.g a Newton Raphson step etc.

#include<stdio.h>
#include<iostream.h>
#include<math.h>
main()
{
float a,b,c,d,k,x,y;
printf("Enter the values of a,b,c:");
scanf("%f%f%f",&a,&b,&c);
d = ((b*b)-(4*a*c));
if(d>=0)
{
k = sqrt(d);
x = (-b+k)/(2*a);
y = (-b-k)/(2*a);
printf("The quadratic equation is x = %f and y = %f",x,y);
}
else
{
float re, im;
d*=-1;
k = sqrt(d);
re = -b/(2*a);
im = k/(2*a);
printf("The quadratic equation is x=%f+(%fi)\n",re,im);
printf("The quadratic equation equation is y=%f-(%fi)\n",re,im);
}
return 0;
}