The Pythagoras Theorem equation is:
a2 + b2 = c2
It is used to calculate an unknown side of a right angled triangle. a, b and c represent sides of the triangle and you can rearange the formula to find whichever side you are looking for.
c2 - b2 = a2
c2 - a2 = b2
Once you have found what a2 or b2 or c2 is you then simply square root that number to find the length of your side.