Answer
$i\sqrt{15}$
Work Step by Step
Using $\sqrt{ab}=\sqrt{a}\cdot\sqrt{b},$ the given expression, $
\sqrt{-15}
,$ is equivalent to
\begin{align*}
&
\sqrt{-1}\cdot\sqrt{15}
.\end{align*}
Since $i=\sqrt{-1},$ the expression above is equivalent to
\begin{align*}\require{cancel}
&
i\cdot\sqrt{15}
\\&=
i\sqrt{15}
.\end{align*}
Hence, the simplified form of the given expression is $
i\sqrt{15}
$.