The idea of the Pythagorean Theorem is that, when you have a right triangle, a squared + b squared = c squared.
a and b are your two base sides, while c is your hypotenuse, or longest side.
An easy example of this is a 3-4-5 triangle. So if you have a right triangle, and one of the base sides is 3, with the other base being 4, and the longest side being 5.
Test it:
3 squared is 9
4 squared is 16
9+16=25
And 25=5 squared.
Understand?