The hypotenuse is the longest side of a triangle and is opposite the right-angle in a right-angled triangle. From what you tell me, Phythagoras theorem shall be used here. Where:
c^2 = a^2 + b^2 (note, a b and c can be any value) in this case the c represents the hypotenuse (its the longest side)
so, you have 3 and 4 as the other lengths. THat means
c^2 = 3^2 + 4^2
= 9 + 16
c = sq. root 25
Happy to help!