you can solve this using pythagoras theorem which states that the sum of the square of the lengths of the sides adjacent to the right angle is equal to the square of the length of ypotenuse (hypotenuse is the longest side which is opposite to right angle)
( the attached image will help you understand the theorem better)
so
20² + 15² = x²
400 + 225 = x²
625 = x²
√625 = x
25 = x
