(-3xy)^2*x^4-2x^2(3xy^2)^2
=9x^2y^2*x^4-2x^2*9x^2y^4
=9x^6y^2-18x^4y^4
=9x^4y^2(x^2-y^2)
=9x^4y^2(x+y)(x-y)