第一项与第三项先合并
9xy(x-y)(x+1)-3y(x-y)(3x+2y)+6y^2*(x-y)(x+1)
=(x-y)(x+1)(9xy+6y^2)-3y(x-y)(3x+2y)
=(x-y)(x+1)(3x+2y)3y-3y(x-y)(3x+2y)
=3y(x-y)(3x+2y)(x+1-1)
=3xy(x-y)(3x+2y)