Check that, but here is how you need to set this up:
Volume = length x width x height
Surface area (box with no top) = 2(length x height) + 2(width x height) + (length x width)
V = 8h
A = 2(4h) + 2(2h) + 8
= 8h + 4h +8
= 12h + 8
Double check the question and then I can help you through the rest. IT really isn't an optimization problem the way it's worded.
Since you are given your volume, you should have two variables in there.