Use the Pythagorean theorem!
This principle applies to any right triangles.
This states the relationship between the sides that
(7)² + (24)² = x²
All you have to do is simplify at this point.
49 + 576 = x²
625 = x²
√ √
x = 25
You should try to memorize this right triangle. It can be very helpful in fast questionnaires, such as the SAT.