Answer
$-2i\sqrt{2}$
Work Step by Step
Using $i=\sqrt{-1}$ and the properties of radicals, the given expression, $
-\sqrt{-8}
,$ simplifies to
\begin{array}{l}\require{cancel}
-\sqrt{-1}\cdot\sqrt{8}
\\\\=
-i\sqrt{4\cdot2}
\\\\=
-i\sqrt{(2)^2\cdot2}
\\\\=
-i\cdot2\sqrt{2}
\\\\=
-2i\sqrt{2}
.\end{array}
