9x^2（m-n)+y^2（n-m）
=9x^2(m-n)-y^2(m-n)
=(m-n)(9x^2-y^2)
=(m-n)(3x+y)(3x-y)
-x^2y+2xy^2-y^3
=-y(x^2-2xy+y^2)
=-y(x-y)^2
3ab^2+a^2b
=ab(a+3b)