For a given girth, the largest area will be that of a square. Then for some length x, the volume will be
.. V = x((108 -x)/4)^2
.. = (1/16)*x*(108 -x)^2
The largest volume will be found where the derivative of this with respect to x is zero.
.. V' = (1/16)*((108 -x)^2 -2x(108 -x))
.. 0 = (108 -x)(108 -x -2x)
.. x = 108 or 108/3 = 36
The dimensions of the largest package that can be mailed are
.. 36 inches in length and 72 inches in girth
The volume of that package is 11,664 in^3.
