The Pythagorean Theorem says, for a right triangle
with leg lengths a and b and hypotenuse c:
a2 + b2 = c2
Given a=3 and b=4
32 + 42 = c2
9 + 16 = c2
25 = c2
Take square root of both sides
√25 = c
5 = c
The length of the hypotenuse is 5.
Pretty straight forward. Got it?