Let's suppose that the box has length l, width w, and height h [note: you should fill in the actual numbers].
There are six rectangular sides of the box:
- areas of top and bottom: 2*w*l
- areas of front and back: 2*l*h
- areas of two side panels: 2*w*h
The total surface area = 2wl + 2lh + 2wh