In order to solve this, we will need to use Pythagoras' Theorem:
[tex] c^{2} = a^{2} + b ^{2} [/tex]
However, since we are trying to find the shorter side, we need to rearrange the formula:
[tex] a^{2} = c^{2} - b^{2} [/tex]
In this case, c = 13in (hypotenuse)
b = 12in
Sub these values to find a,
[tex] a^{2} = 13^{2} - 12^{2} [/tex]
[tex] a^{2} [/tex] = 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 :)