\[128x^{2}\ y^{3} \cdot \left( - \frac{1}{4}\text{\ x}y^{5}\ \right)^{3} =\]
\[= 128x^{2}y^{3} \cdot \left( - \frac{1}{64}x^{3}y^{15} \right) = - 2x^{5}y^{18}\]