7. $$ \frac{5y}{y^2-9} - \frac{15}{y^2-9} = \frac{5y-15}{y^2-9} = \frac{5(y-3)}{(y-3)(y+3)} = \frac{5}{y+3} $$
Ответ: $$ \frac{5}{y+3} $$