Question 156384
If the square ends of the of the box are 18 inches on each side, then the girth of the box is 4*18 = 72 inches.
To meet the requirements for priority mail, the girth (72 inches) plus length (L) must be no more than 108 inches, so you can write:
{{{L+72 <= 108}}} Subtract 72 from both sides.
{{{L <= 36}}}inches.
The length is no more than 36 inches.
The maximum volume of such a box would be:
{{{V = B*L}}} Where: B = area of the base (or end) of the box, and this is: {{{B = 18^2}}}
{{{B = 324}}} sq.in., so the maximum volume is:
{{{V = (324)(36)}}} 
{{{V = 1164}}}Cubic inches.