#### Answer

$3\sqrt{3}$

#### 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 $3$ units and the hypotenuse is $6$ units.
Using $a^2+b^2=c^2$ or the Pythagorean Theorem, with $
a=3
$ and $
c=6
,$ then
\begin{array}{l}\require{cancel}
a^2+b^2=c^2
\\\\
3^2+b^2=6^2
\\\\
9+b^2=36
\\\\
b^2=36-9
\\\\
b^2=27
\\\\
b=\sqrt{27}
\\\\
b=\sqrt{9\cdot3}
\\\\
b=\sqrt{(3)^2\cdot3}
\\\\
b=3\sqrt{3}
.\end{array}
Hence, the missing side measures $
3\sqrt{3}
$ units.