No, I got up to what you wrote in reply to my thread. That part I understand. I ran into problems when I was taking the partial derivative and was hoping you could show me how to take the partial derivatives. Thanks for your reply though and I'm hoping you can go a couple of steps further.

By the way the book gives the answer 3/7x + 2/7y + 6/7z. It appears the point (3, 2, 6) is used in the answer but I don't know how they got it.