#### Answer

$-28$

#### Work Step by Step

Using $i=\sqrt{-1},$ $i^2=-1,$ and the properties of radicals, the given expression, $
\sqrt{-49}\sqrt{-16}
,$ simplifies to
\begin{array}{l}\require{cancel}
\sqrt{-1}\sqrt{49}\cdot\sqrt{-1}\sqrt{16}
\\\\=
i\sqrt{49}\cdot i\sqrt{16}
\\\\=
i\sqrt{(7)^2}\cdot i\sqrt{(4)^2}
\\\\=
7i\cdot 4i
\\\\=
28i^2
\\\\=
28(-1)
\\\\=
-28
.\end{array}