```
import math
print "Select unknown variable"
print "a, b, or c"
print
equation=input("> ")
if equation == a:
b=input("Enter b: ")
c=input("Enter c: ")
a=math.sqrt(c ** 2 - b ** 2)
print "a= ",a
```

**i get the following traceback

Traceback (most recent call last):

File "C:\Python27\My Programs\pt.py", line 7, in <module>

equation=input("> ")

File "<string>", line 1, in <module>

NameError: name 'a' is not defined

they say a is not defined but i dont see why i have to if im using "==", and im using python 2.7