Answer
$7\sqrt{2}$
Work Step by Step
The diagonal and the two sides of the square form a right triangle with the legs equal to $7.$
Using $a^2+b^2=c^2$ or the Pythagorean Theorem, with $
a=7
$ and $
b=7
,$ then
\begin{array}{l}\require{cancel}
a^2+b^2=c^2
\\\\
7^2+7^2=c^2
\\\\
49+49=c^2
\\\\
c=\sqrt{49+49}
\\\\
c=\sqrt{49(2)}
\\\\
c=\sqrt{(7)^2\cdot2}
\\\\
c=7\sqrt{2}
.\end{array}
Hence, the missing side measures $
7\sqrt{2}
$ units.
