The surface area equation for the box below is 2x^2 + 20x.
Just set 2x2 + 20x = 112
Divide through by 2
x2 + 10x = 56
Rearrange
x2 + 10x - 56 = 0
This factors as
(x + 14)(x - 4) = 0
The only positive solution is x = 4 (inches).
