Question 806878
The equation for a box problem will most likely be x(length-2x)(width-2x) unless stated otherwise.
{{{x(18-2x)(42-2x)}}}
To find the max you plug in numbers for x that are {{{0<x<9}}}.  
The reason why you can't use 0 or 9 is because if you plug those numbers into the equation you get out 0.  You can't have a volume of 0.
You would usually use a graphing calculator for these problems because it takes forever using a regular calculator.  
Your max will be 3.917 with a volume of 1360.497705.