Answer
$3\sqrt{5}$
Work Step by Step
Using $\sqrt{x}\cdot\sqrt{y}=\sqrt{xy}$ or the product rule of radicals, the given expression, $
\sqrt{3}\cdot\sqrt{15}
,$ is equivalent to
\begin{array}{l}\require{cancel}
\sqrt{3(15)}
\\\\=
\sqrt{3(3\cdot5)}
\\\\=
\sqrt{(3)^2\cdot5}
\\\\=
3\sqrt{5}
.\end{array}
