Помогите пожалуйста с 5ым номером

Решите задачу:

$\frac{21-5y}{y^2-9}- \frac{y}{y^2-9}: \frac{y}{y+3}-\frac{y-3}{y+3} = -1\\ \\ 1)\frac{y}{y^2-9}: \frac{y}{y+3}=\frac{y}{(y-3)(y+3)}* \frac{(y+3)}{y}= \frac{1}{y-3} \\ \\ 2) \frac{21-5y}{y^2-9}- \frac{1}{y-3} -\frac{y-3}{y+3}= \frac{21-5y-1(y+3)-(y-3)(y-3)}{(y-3)(y+3)}= \\ \\ = \frac{21-5y-y-3-(y-3)^2}{(y-3)(y+3)}=\frac{21-5y-y-3-(y^2-6y+9)}{(y-3)(y+3)}= \\ \\ =\frac{21-5y-y-3-y^2+6y-9}{(y-3)(y+3)}=\frac{9-y^2}{(y-3)(y+3)}=\frac{-(y^2-9)}{(y-3)(y+3)}=\frac{-(y-3)(y+3)}{(y-3)(y+3)}=-1$