#### Answer

$\sqrt{21xy}$

#### Work Step by Step

Using the Product Rule of radicals which is given by $\sqrt[m]{x}\cdot\sqrt[m]{y}=\sqrt[m]{xy},$ then
\begin{array}{l}\require{cancel}
\sqrt{7x}\cdot\sqrt{3y}
\\\\=
\sqrt{7x(3y)}
\\\\=
\sqrt{21xy}
.\end{array}