How can we make a box with maximum volume from a given size "blank" of material? (This is a modification of Dani Novak's excellent demo)

You are given a "blank" of cardboard and asked to make a box out of it, that has the most possible volume. You make the box by cutting squares of side [math]x[/math] from the corners, then folding the "flaps" up to form sides. (The top stays open).[br]