\[(a - 5)(a + 1) - (a - 6)(a - 1) =\]
\[= a^{2} + a - 5a - 5 - \left( a^{2} - a - 6a + 6 \right) =\]
\[= a^{2} - 4a - 5 - a^{2} + a + 6a - 6 =\]
\[= 3a - 11.\]