Is part a correct? and can someone me with part b? (highlighted)

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.