6. $$\frac{y-3}{y+3}$$ $$\cdot$$ (y + $$\frac{y^2}{3-y}$$) = $$\frac{y-3}{y+3}$$ $$\cdot$$ $$\frac{3y-y^2+y^2}{3-y}$$ = $$\frac{(y-3)3y}{(y+3)(3-y)}$$ = $$\frac{-3y(3-y)}{(y+3)(3-y)}$$ = $$\frac{-3y}{y+3}
Ответ: $$\frac{-3y}{y+3}$$