\[5a^{3}x^{2}y^{2} - 15a^{3}xy^{2} - 5a^{4}y =\]
\[= 5a^{3}y \cdot \left( x^{2}y^{2 - 1} - \frac{15}{5}xy^{2 - 1} - a^{4 - 3} \right) =\]
\[= 5a^{3}y \cdot \left( x^{2}y - 3xy - a \right)\text{.\ }\]