Arectangular tank that is 87888788 ft cubedft3 with a square base and open top is to be constructed of sheet steel of a given thickness. find the dimensions of the tank with minimum weight.

We assume the material has the same thickness for sides and bottom. In that case, the problem is the same as minimizing the surface area of the box. (See link for a discussion of this problem in general.)

The dimensions of the edge of the base are
  ∛(2×8788 ft³) = 26 ft

The box is 26 ft square and 13 ft high.