6) ab-36-2a+6 15-5a

6) $$ \frac{ab - 3b - 2a + 6}{15 - 5a} = \frac{b(a - 3) - 2(a - 3)}{5(3 - a)} = \frac{(a - 3)(b - 2)}{-5(a - 3)} = \frac{b - 2}{-5} = -\frac{b - 2}{5} = \frac{2 - b}{5} $$

Ответ: $$\frac{2-b}{5}$$