x^6-x^4y^2+3x^2y^3-3x^4y
=(x^6-x^4y^2)-(3x^4y-3x^2y^3)
=x^4(x^2-y^2)-3x^2y(x^2-y^2)
=(x^4-3x^2y)(x^2-y^2)
=x^2(x^2-3y)(x+y)(x-y)