Question

Given 50meter fencing material to build a square box but only applied to 3 fences (another fence build by concrete wall). But to have maximum Box area.

Answer 1

let each side be x metres
so 3x = 50
x = 50/3

Area = x^2
= (50/3)^2 m^2
= appr 277.8 m^2

since you insist that it must be a square, the area is not variable.
There is no maximum area, it is simply (50/3)^2

Answer 2

as always in this kind of problem, area is maximized when the fencing is divided equally between lengths and widths.
So maximum area is 25 * 12.5 = 312.5

In case you meant rectangular, not literally square.