Answer
5
Work Step by Step
Since we have a right triangle with legs $a$, $b$ and the hypotenuse $c$, then by the Pythagorean theorem, we have $$c^2=a^2+b^2.$$
Thus, the hypotenuse is given by
$$
c=\sqrt{a^2+b^2}=\sqrt{4^2+3^2}=\sqrt{25}=5
.$$