You have to use pythagorean theorem to solve this problem. The pythagorean theorem is: a² + b² = c².
In this case, the third side is equal to c. Side c is always opposite of the right angle. It doesn't matter whether 60 or 45 is a or b because you are going to add them together anyways.
60² + 45² = c²
c² = 5625
c = 75 ...... You get 75 by taking the square root of 5625.
