Maximise surface area of cuboid given constant volume.

Hopefully this is the right section to be putting this ...

A cuboid tank is open at the top and the internal dimensions of its base are x m and 2x m. The height is h m. The volume of the tank is V cubic metres and the volume is fixed. Let S m^2 denote the internal surface area of the tank.

Find S in terms of x and h; and V and x.

State the maximal domain for the function defined by the rule in the above question.

If 2 < x < 15 find the maximum value of S if V=1000m^3.

Thank you.