Given that V = xyz(r^−7), find the first partial derivative of V (wrt x) and deduce that the second partial derivative of V (wrt to x) is

−21xyz(r^−9) + (63x^3)yz(r^−11)

NB: d = delta sign, del = del function (upside down triangle)

I've tried it several times now. I can see where the coefficents will eventually come from being multiples of 7, but I can't get the answer above. Can anyone help? Thanks.