The pythagorean theorem, a squared + b squared = c squared, is used to find missing side lengths in a right triangle. solve for b, in terms of c and a.

C2 -a2 =b2 

if a triangle is 20ft high with the hypotenuse 25ft 
                       a2                                           c2
                                        23.5ft
                                         b2    
 
 i don't do this traditionally  i work it a very long way thats easier for me but this is my go to ex. that i show people and it is correct and easy to visualize 
but i hope it helps you some