\[\ \frac{3a - 9}{a + 2}\ :\frac{a^{2} - 9}{a^{2} - 4} =\]
\[= \frac{3 \cdot (a - 3)(a - 2)(a + 2)}{(a + 2)(a - 3)(a + 3)} = \frac{3a - 6}{a + 3}\]