\[3a\left( 3a - a^{2} \right) - 4a\left( 2a^{2} - 5a \right) =\]
\[= 9a^{2} - 3a^{3} - 8a^{3} + 20a^{2} =\]
\[= - 11a^{3} + 29a²\]