Question 718019
Original side length of the square sheet, call it variable L.  The bottom, when the 6 foot square piece is removed and flaps raised, will be {{{(L-2)*6}}}.  Since this started with a square sheet, each side is 6 feet high.


Volume which this box holds is the area of the base multiplied by height.
Volume is {{{(L-12)^2*6=864}}}.  A little more simply, {{{6(L-12)^2=864}}}, and we should simplify by division by 6: {{{highlight((L-12)^2=144)}}}, which should be uncomplicated to solve for L.