\[- 6 \cdot (a + 4)(a - 1) =\]
\[= - 6 \cdot \left( a^{2} - a + 4a - 4 \right) =\]
\[= - 6 \cdot \left( a^{2} + 3a - 4 \right) =\]
\[= - 6a^{2} - 18a + 24.\]