First, we need to find the length of the unknown side.
The formula to find the hypotenuse is given by:
c^2 = a^2 + b^2
BUT! Since we need to find the length of the shorter side, we rearrange the formula to:
a^2 = c^2 - b^2
Let the unknown side be x,
x^2 = 13^2 - 5^2
x^2 = 144
Square root both sides to get x,
x = 12
Now that you have x, which is the height of the triangle, you need to use the formula to find the area, given by:
A = 1/2 x b x h
In this case, b = 5, and h = 12. Sub these values into the formula:
A = 1/2 x 5 x 12
= 30
Hope this helped! Ask me if there's any part of the working you don't understand :)