г) $$\frac{6a^2-a-1}{8a+b-2ab-4} = \frac{6a^2-a-1}{8a-2ab+b-4} = \frac{6a^2-a-1}{2a(4-b)-(4-b)} = \frac{6a^2-a-1}{(2a-1)(4-b)}$$