(x^2-9y^2)(x^2+3xy+9y^2)(x^2-3xy+9y^2)
=(x+3y)(x-3y)(x^2+3xy+9y^2)(x^2-3xy+9y^2)
=[(x+3y)(x^2-3xy+9y^2)][(x-3y)(x^2+3xy+9y^2)]
=(x^3+27y^3)(x^3-27y^3)
=x^6-729y^6