\[\left( - \frac{3xy^{3}}{a^{4}} \right)^{2} \cdot \frac{a^{8}}{27x^{3}y^{5}} =\]
\[= \frac{9x^{2}y^{6} \cdot a^{8}}{a^{8} \cdot 27x^{3}y^{5}} = \frac{y}{3x}.\]