#### Answer

$-18$

#### Work Step by Step

Using $i=\sqrt{-1},$ $i^2=-1,$ and the properties of radicals, the given expression, $
\sqrt{-36}\sqrt{-9}
,$ simplifies to
\begin{array}{l}\require{cancel}
\sqrt{-1}\sqrt{36}\cdot\sqrt{-1}\sqrt{9}
\\\\=
i\sqrt{(6)^2}\cdot i\sqrt{(3)^2}
\\\\=
6i\cdot3i
\\\\=
18i^2
\\\\=
18(-1)
\\\\=
-18
.\end{array}