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