\[8 - (2 + a)(3a + 4) =\]
\[= 8 - \left( 6a + 8 + 3a^{2} + 4a \right) =\]
\[= 8 - 3a^{2} - 10a - 8 =\]
\[= - 3a^{2} - 10a\]