The surface area of a box which is in the form of a cuboid whose dimensions are
l x b x h is:
2(lb + bh + hl)
The surface area of a cuboid is nothing but the total surface area which is the area of the 6 rectangular faces {(l,b), (b,h), (l,h), (l,b), (b,h), (l,h)}.
Total surface area = lb + bh + lh + lb + bh + lh = 2(lb + bh + hl)