How do I use Pythagorus' Theorum?

Pythagorus' Theorum is used to calculate the lengths of the sides of a right angled triange, and is written mathematically as follows:

a² + b² = c²

The letters (a, b and c) each stand for one of the three sides. Where c represents the hypotenuse, which is the side opposite to the right angle, and is usually the longest side of the triangle. 

The theorum therefore means that the square of the hypotenuse (c) is always equal to the sum of the square of the other two sides (a and b).

This can be used to find the length of any side of a right angled triangle as long as the other two sides are already known.