To find any side of a right angled triangle, we use Pythagorean Theorem. It states that the square of hypotenuse is equal to the sum of squares of it's sides.
Let hypotenuse is a
Perpendicular is b
and base is c
According to the theorem:
a²= b² + c²
Add values and solve for the unknown variable / side.
If you had added the problem, I would have elaborated more but still this will help you out.
