Answer
$\sqrt{113} \text{ } in.$
Work Step by Step
Using $a^2+b^2=c^2$ (or the Pythagorean Theorem) where the legs are given the measures $7$ inches and $8$ inches, then the hypotenuse, $c$, is equal to
\begin{array}{l}\require{cancel}
7^2+8^2=c^2
\\\\
49+64=c^2
\\\\
113=c^2
\\\\
c=\sqrt{113}
.\end{array}
Hence, the unknown side is $
\sqrt{113} \text{ } in.
$
