в) $$ \frac{3a - 36}{12b - ab} = \frac{3(a - 12)}{b(12 - a)} = \frac{-3(12 - a)}{b(12 - a)} = \frac{-3}{b} = -\frac{3}{b} $$
Ответ: $$\frac{-3}{b}$$