The Pythagorean theorem applies. If the sides are designated by "a" and "b", and the hypotenuse is designated by "c", then that theorem tells you
c² = a² + b²
Filling in your numbers, we can solve for b.
74² = 24² + b²
74² - 24² = b² = 4900
b = √4900 = 70
The length of the third side is 70.