volume of box = (width)(length)(height)
width = 15 - x - x
width = 15 - 2x
length = 22 - x - x
length = 22 - 2x
height = x
volume of box = (width)(length)(height)
volume of box = (15 - 2x)(22 - 2x)x
The expression (15 - 2x)(22 - 2x)x can be used to determine the volume of the box.
( And by looking at a graph we can see that the greatest possible volume is about 432 in3 .
Here's a graph: https://www.desmos.com/calculator/h7gylso5p9
And note that the length of x can't actually be greater than 15/2 in. )