6x(x-y)^2+3(y-x)^3
＝6x(x-y)^2+3(y-x)(y-x)^2
＝6x(x-y)^2-3(x-y)(x-y)^2
＝[6x-3(x-y)](x-y)^2
＝(6x-3x+3y)(x-y)^2
＝(3x+3y)(x-y)^2
＝3(x+y)(x-y)^2