\[5 \cdot \left( 3x^{3} - x^{2} - ax + a^{3} \right) \cdot a =\]
\[= 5a\left( 3x^{3} - x^{2} - ax + a^{3} \right) =\]
\[= 15ax^{3} - 5ax^{2} - 5a^{2}x + 5a^{4}.\]