a^3*(-ab^3)^3
=a^3*(-a)^3*(b^9)
=-a^6*b^9
(-x^3y^3)(7xy^2-9x^2y)
=-(xy)^3*xy*(7y-9x)
=-(xy)^4*(7y-9x)
=9x^5y^4-7x^4y^5