Pythagoras theorum is used to identify the lengths of any side on a right angle triangle. The theorem is as follows:
a2 + b2 = c2
We assume that the hypotenuse (side opposite the right angle) is C. Therefore, to find the value for side C, we need to add the squares of the other sides and then square root. (I would use a diagram of a triangle to explain this in the Learning Space)