1. It looks like the picture pretty much shows the cross-section as being a square
2. We have that G + L = 108 ⇒ G = 108 - L
Assuming that the cross-section is a square....its side is 1/4 of the girth and can be expressed as:
G/4 = (108 - L) / 4
So....the volume can be expressed as L [ (108 - L) / 4] ^2
So we have that
V = L [ (108 - L) / 4]^2 = L [ 27 - L/4]^2 = L [ L^2/16 - 27L/2 + 729 ] =
L^3/16 - 27L^2//2 + 729L
To find the max volume, we can use some Calculus or a graph
The graph seems easiest....here it is :
https://www.desmos.com/calculator/ybfbga97jl
3. Looking at the graph, the maximum volume is [ again, assuming a square base ] = 11664 in^3
4. The Length, L = 36 in
The Girth, G = 108 - 36 = 72 in
So....the side of the square is 1/4 of this = 18 in
So...the dimensions are 18 in x 18 in x 36 in = 11664 in^3
Note that the restrictions have been met Girth + Length = (4 * 18) + 36 = 72 + 36 = 108 in