Answer
$\frac{y\sqrt[4] {xy^{2}}}{3}$
Work Step by Step
$\sqrt[4] \frac{xy^{6}}{81}$
=$\sqrt[4] \frac{x \times y^{4}\times y^{2}}{3^{4}}$
=$\frac{\sqrt[4] {xy^{2}}\times \sqrt[4] {y^{4}}}{\sqrt[4] {3^{4}}}$
=$\frac{\sqrt[4] {xy^{2}}\times y}{3}$
=$\frac{y\sqrt[4] {xy^{2}}}{3}$
