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

= 25

c = sq. root 25

= 5

Happy to help!