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