dy=d(x^2)-2d(xsinx^3)

=2xdx-2(sinx^3)dx-2xd(sinx^3)
=(2x-2sinx^3)dx-2xcosx^3d(x^3)
=(2x-2sinx^3)dx-2xcosx^3·3x^2dx
=(2x-2sinx^3-6x^3·cosx^3)dx