140 x 120 gives the surface area of the top which is also the surface area of the bottom.
16800 x 2 = 33600
140 x 4 will give the surface area of two more sides
560 x 2 = 1120
120 x 4 will give the final 2 sides
480 x 2 = 960
add everything together to get the total surface area
35680 square mm