в) $$\frac{4a^2-1}{a^2-9} : \frac{6a+3}{a+3} = \frac{(2a-1)(2a+1)}{(a-3)(a+3)} : \frac{3(2a+1)}{a+3} = \frac{(2a-1)(2a+1)}{(a-3)(a+3)} \cdot \frac{a+3}{3(2a+1)} = \frac{2a-1}{3(a-3)}$$
Ответ: $$\frac{2a-1}{3(a-3)}$$