the answer here is 3.
basically for the pythagorean theorem you use the formula a^2 + b^2 = c^2
it works for right triangles, with a and being being sides of the triangles and c being the hypotenuse
therefore for this specific example, you plug in the side 4 and the hypotenuse 5:
a^2 + 4^2 = 5^2
a^2 + 16 = 25
a^2 = 9
a = 3
