Answer
$\frac{y}{3x}$
Work Step by Step
$\sqrt[4] (\frac{y^{4}}{81x^{4}})=\frac{y}{3x}$, because $(\frac{y}{3x})^{4}=\frac{y}{3x}\times\frac{y}{3x}\times \frac{y}{3x}\times \frac{y}{3x}=\frac{y^{1+1+1+1}}{(3\times3\times3\times3)\times x^{1+1+1+1}}=\frac{y^{4}}{81x^{4}}$
