The Pythagorean theorem is a squared + b squared = c squared. It is used for right triangles with a and b being both legs and c being the hypotenuse. If you have the length of any two legs you can find the length of the third.
In a right triangle a^2+b^2=c^2. C is the hypotenuse(side across from the right angle). You can substitute the given length of any two side to find the other side.
