\[\frac{16 - a^{2}}{a^{2} - 3a}\ :\frac{a^{2} + 4a}{a^{2} - 9} =\]
\[= \frac{16 - a^{2}}{a^{2} - 3a} \cdot \frac{a^{2} - 9}{a^{2} + 4a} =\]
\[= \frac{(4 - a)(4 + a)(a - 3)(a + 3)}{a(a - 3)a(a + 4)} =\]
\[= \frac{(4 - a)(a + 3)}{a^{2}}.\]