в) $$ \frac{3a-9}{a+2} : \frac{a^2-9}{a^2-4} = \frac{3(a-3)}{a+2} : \frac{(a-3)(a+3)}{(a-2)(a+2)} = \frac{3(a-3)}{a+2} \cdot \frac{(a-2)(a+2)}{(a-3)(a+3)} = \frac{3(a-3)(a-2)(a+2)}{(a+2)(a-3)(a+3)} = \frac{3(a-2)}{a+3} $$
Ответ: $$ \frac{3(a-2)}{a+3} $$