#### Answer

The length of the unknown side is $\sqrt{113}$.

#### Work Step by Step

In the formula $a^2 + b^2 = c^2$, $c$ is the hypotenuse. Here, $a = 7$ and $b = 8$. We solve for c, the hypotenuse. Then $a^2 + b^2 = c^2$ becomes
$7^2 +8^2 =c^2$
$49+64 =113$
$c^2 = 113$
$c = \sqrt{113}
No simplification possible.