In order to solve this, we will need to use Pythagoras' Theorem:

However, since we are trying to find the shorter side, we need to rearrange the formula:

In this case, c = 13in (hypotenuse)
b = 12in
Sub these values to find a,


= 25
Square root both sides to get 'a' on its own,
a = 5
Therefore, the unknown side is 5in!
Hope this helped! Ask me if there's any part of the working you don't understand :)