Answer
$\sqrt{21xy}$
Work Step by Step
Using $\sqrt{x}\cdot\sqrt{y}=\sqrt{xy}$ or the product rule of radicals, the given expression, $
\sqrt{7x}\cdot\sqrt{3y}
,$ is equivalent to
\begin{array}{l}\require{cancel}
\sqrt{7x(3y)}
\\\\=
\sqrt{21xy}
.\end{array}
Note that variables are assumed to have positive real numbers.
