Answer
$\dfrac {3}{8r}$
Work Step by Step
$\dfrac {3r^{3}-9r^{2}}{r^{2}-9}:\dfrac {8r^{3}}{r+3}=\dfrac {3r^{3}-9r^{2}}{r^{2}-9}\times \dfrac {r+3}{8r^{3}}=\dfrac {3r^{2}\left( r-3\right) }{\left( r-3\right) \left( r+3\right) }\times \dfrac {r+3}{8r^{3}}=\dfrac {3}{8r}$
