Answer
$\text{approximately } 10.0 \text{ } mm.$
Work Step by Step
Using $a^2+b^2=c^2$ (or the Pythagorean Theorem) where the legs are given the measures $7$ millimeters and $7.2$ millimeters, the measure of the hypotenuse, $c$, is equal to
\begin{array}{l}\require{cancel}
(7)^2+(7.2)^2=c^2
\\\\
49+51.84=c^2
\\\\
100.84=c^2
\\\\
c=\sqrt{100.84}
\\\\
c\approx10.0
.\end{array}
Hence, the unknown side is $
\text{approximately } 10.0 \text{ } mm.
$
