Question

prove that 2(lb+bh+hl )

Answer 1

total surface area of all the faces of a cuboidal  box = h x l +b x l + b x h + l x h +b x h + l x b = 2 (h x l +b x h+ b x l) = 2 (lb + bh+ hl)

hope my answer helps you:)

Answer 2

if we observe a cuboid
we will see that 
top and bottom =lb+lb=2lb
front and back=bh+bh=2bh
left and right=lh+lh=2lh

so,
2lb+2bh+2lh=2(lb+bh+lh)