#### Answer

$6y^{2}\sqrt {2xy}$

#### Work Step by Step

Breaking the radicand into its factors and simplifying, we obtain:
$\sqrt {72xy^{5}}$
=$\sqrt {2\times36\times x \times y^{4}\times y}$
=$\sqrt {36\times y^{4}}\times\sqrt {2\times x\times y}$
=$\sqrt {6^{2}\times (y^{2})^{2}}\times\sqrt {2\times x\times y}$
=$6\times y^{2}\times\sqrt {2\times x\times y}$
=$6y^{2}\times\sqrt {2xy}$
=$6y^{2}\sqrt {2xy}$