Part A. Your model is as good as any. It is hard to tell if the label needs to include the descriptor (length = 12 ft, for example). Certainly your model is sufficient for Part B.
Part B. The total area is the sum of the areas of each of the 6 faces of the box. Opposite faces are the same area, so you have
A = 2(LW +WD +LD) = 2(LW +D(L +W))
Subsituting the given dimensions, you get
A = 2((12 ft)(6 ft) +(3 ft)(12 ft +6 ft))
A = 2(72 ft² +(3 ft)(18 ft))
A = 2(72 ft² +54 ft²) = 252 ft²
The least amount of paper required to cover the box is 252 ft².