Answer
$\sqrt{33} \text{ } cm.$
Work Step by Step
Using $a^2+b^2=c^2$ (or the Pythagorean Theorem) where a leg is given the measure $4$ centimeters and the hypotenuse is given the measure $7$ centimeters, then the other leg, $b$, is equal to
\begin{array}{l}\require{cancel}
4^2+b^2=7^2
\\\\
b^2=7^2-4^2
\\\\
b^2=49-16
\\\\
b^2=33
\\\\
b=\sqrt{33}
.\end{array}
Hence, the unknown side is $
\sqrt{33} \text{ } cm.
$