Total surface area of a cuboid having dimensions $l\times b\times h$ is ____

A cuboid's total surface area is equal to the sum of the surfaces of the six rectangles that make up the cuboid.
Three pairs of rectangles with opposite sides and the same surface area are formed by the six rectangles.
Area of a single rectangle pair with sides $l$ and $b$ is = $2(l\times b)$.
The second set of rectangles with sides b and h have an area = $2(b\times h)$.
The third pair of rectangles with sides l and h have an area = $2(l\times h)$.
The cuboid's total area = the sum of the three pairs of rectangles 
= $2(l\times b)+2(b\times h)+2(l\times h)$
= $2(lb+bh+hl)$.