#### Answer

$5\sqrt{3}$

#### Work Step by Step

The given triangle is an equilateral triangle. In an equilateral triangle the height bisects the base into two equal parts. Hence, a right triangle is formed where one of the legs is $5$ units and the hypotenuse is $10$ units.
Using $a^2+b^2=c^2$ or the Pythagorean Theorem, with $
a=5
$ and $
c=10
,$ then
\begin{array}{l}\require{cancel}
a^2+b^2=c^2
\\\\
5^2+b^2=10^2
\\\\
25+b^2=100
\\\\
b^2=100-25
\\\\
b^2=75
\\\\
b=\sqrt{75}
\\\\
b=\sqrt{25\cdot3}
\\\\
b=\sqrt{(5)^2\cdot3}
\\\\
b=5\sqrt{3}
.\end{array}
Hence, the missing side measures $
5\sqrt{3}
$ units.