The Pythagorean Theorem is one of the most important theorems in geometry.  It allows us to find the length of 1 side of a right-angled triangle when given 2 others.  Simply put,

c ² = a² + b ^{2 
where, c is the length of the longest side (or hypotenuse)}

Don't take my word for it.  Prove to yourself that this formula works for any right-angle triangles of any size.  

Let's take this a step further..