Find the minimum & maximum area of a box!

Here's another question Ive come across during revision

http://i55.tinypic.com/2poxs36.png

So

Volume is

Total area is base of box + 4 sides -

And then I got the derivative of

And where this derivative = 0, there was a local minimum. This was where w = 40, so that makes d = 40 and h = 20 for the dimensions of the box that will use the minimum amount of material.

I take it this is all correct?

If it is, what I'm wondering is how I would be able to find the dimensions that would use the __maximum__ amount of material? I only got one critical point when differentiating and that turned out to be a local minmum so how is it possible to find the maximum amount of material used?