\[5a - (a + 1)(4a + 1) =\]
\[= 5a - \left( 4a^{2} + a + 4a + 1 \right) =\]
\[= 5a - 4a^{2} - a - 4a - 1 =\]
\[= - 4a^{2} - 1.\]