#### Answer

$-2i\sqrt{7}$

#### Work Step by Step

Using the properties of radicals and that $i=\sqrt{-1}$, then the given expression, $
-\sqrt{-28}
,$ is equivalent to
\begin{array}{l}\require{cancel}
-\sqrt{-1}\cdot\sqrt{28}
\\\\=
-i\cdot\sqrt{4\cdot7}
\\\\=
-i\cdot\sqrt{(2)^2\cdot7}
\\\\=
-i\cdot2\sqrt{7}
\\\\=
-2i\sqrt{7}
.\end{array}