Answer
$3i\sqrt{5}$
Work Step by Step
Using $i=\sqrt{-1}$ and the properties of radicals, the given expression simplifies to
\begin{array}{l}\require{cancel}
\sqrt{-45}
\\\\=
\sqrt{-1}\cdot\sqrt{45}
\\\\=
i\sqrt{9\cdot5}
\\\\=
i\sqrt{(3)^2\cdot5}
\\\\=
3i\sqrt{5}
.\end{array}
