\[(a + 3)(b + 5) - (a + 3)(b + 6) =\]
\[= (a + 3)\left( b + 5 - (b + 6) \right) =\]
\[= (a + 3)(b + 5 - b - 6) =\]
\[= (a + 3) \cdot ( - 1) = - (a + 3).\]