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