\[\frac{5y}{y^{2} - 9} - \frac{15}{y^{2} - 9} = \frac{5y - 15}{y^{2} - 9} =\]
\[= \frac{5 \cdot (y - 3)}{(y - 3)(y + 3)} = \frac{5}{y + 3}\]