\[\left( 3x^{6}y^{3} \right)^{4} \cdot \left( - \frac{1}{81}xy^{2} \right) =\]
\[= 81x^{24}y^{12} \cdot \left( - \frac{1}{81}xy^{2} \right) =\]
\[= - x^{25}y^{14}.\]