(x-2y)(1+x+2y+4y^2+2xy+x^2)
=(x-2y)[(1+(x+2y)+(4y^2+2xy+x^2)]
=(x-2y)+(x-2y)(x+2y)+(x-2y)(4y^2+2xy+x^2)
=x-2y+x^3-y^2+x^3-y^3