In this case, you would use Pythagoras' Theorem:
a^2 = b^2 + c^2
with a being the hypotenuse (a is always the hypotenuse I'm Pythagoras' Theorem).
So we substitute the other sides in (it doesn't matter which side is b or c):
a^2 = 3^2 + 4^2
= 9 + 16
= 25
Therefore:
a = 5
Because in order to get rid of the squared on the side with the a, you must square foot both sides. This will get rid of the squared on the left and square the right hand side. The root of 25 is 5 and so a = 5
