\[- 3a^{4} - 12a^{3} - 12a^{2} =\]
\[= - 3a^{2}\left( a^{2} + 4a + 4 \right) =\]
\[= - 3a^{2}(a + 2)^{2} =\]
\[= - 3a²(a + 2)(a + 2)\]