Answer
$-9m\sqrt{m}$.
Work Step by Step
The given expression is
$=-\sqrt{81m^3}$
Factor as square terms.
$=-\sqrt{81m^2\cdot m}$
Use product property of square roots.
$=-\sqrt{81}\cdot \sqrt{m^2}\cdot \sqrt{m}$
Use $81=9^2$.
$=-\sqrt{9^2}\cdot \sqrt{m^2}\cdot \sqrt{m}$
Simplify.
$=-9m\sqrt{m}$.
