#### Answer

$i\sqrt{19}$

#### Work Step by Step

Using the properties of radicals and that $i=\sqrt{-1}$, then the given expression, $
\sqrt{-19}
,$ is equivalent to
\begin{array}{l}\require{cancel}
\sqrt{-1}\cdot\sqrt{19}
\\\\=
i\cdot\sqrt{19}
\\\\=
i\sqrt{19}
.\end{array}