The formula for the surface area is expressed as
A = 2bh + 2lw + 2hl
To solve for l, we would make the terms containing l to stand alone on one side of the equation. It becomes
2hl + 2lw = A - 2bh
We would factorise l on the left hand side of the equation. it becomes
l(2w + 2h) = A - 2bh
Dividing both sides of the equation by 2h + 2w, it becomes
l = (A - 2bh)/(2w + 2h)
The correct option is A
