Of course, the easy solution to the standard box problem...

If the height of the box is h, then the volume of the box is

Input := 


BoxVol[h_] = h (8.5 - 2h) (11 - 2 h);

To find the height (and other dimensions) of the box of maximum volume, we can set the derivative equal to zero

Input := 


sol = Solve[BoxVol'[h]==0,h]

Output =


{{h -> 1.58542}, {h -> 4.91458}}

Input := 


ht = sol[[1,1,2]];

Input := 


BoxVol[ht]

Output =


66.1482