#### Answer

$2i\sqrt{5}$

#### Work Step by Step

Using the properties of radicals and that $i=\sqrt{-1}$, then the given expression, $
\sqrt{-20}
,$ is equivalent to
\begin{array}{l}\require{cancel}
\sqrt{-1}\cdot\sqrt{20}
\\\\=
i\cdot\sqrt{4\cdot5}
\\\\=
i\cdot\sqrt{(2)^2\cdot5}
\\\\=
i\cdot2\sqrt{5}
\\\\=
2i\sqrt{5}
.\end{array}