\[(a + 2)\left( a^{2} - a - 3 \right) =\]
\[= a^{1 + 2} - a^{1 + 1} - 3a + 2a^{2} - 2a - 6 =\]
\[= a^{3} - a^{2} - 5a + 2a^{2} - 6 =\]
\[= a^{3} + a^{2} - 5a - 6.\]