\[125x^{3}y^{4} \cdot \left( - \frac{1}{5}x^{2}y \right)^{3} =\]
\[= 125x^{3}y^{4} \cdot \left( - \frac{1}{125}x^{6}y^{3} \right) = - x^{9}y^{7}\]