Can you guys help me out with it? i try to solve it but my answer is so weird that i think im wrong...

Question- Someone want to build cardboard box with rectangular base. Knowing thatthe rectangle of the base (and therefore the cover) is such that its width is twice its length, determine an dimensions of greater volume that can be built using 2 square meters of cardboard.

I frist make sure what i need to optimize, the volume. Then i try to get "h" by taking the total area of the box, putting that "h" on volume formula and deriving it i would get a nice result, that doesn't happened...