Answer:
V(x) = l*w*x  - 2*l*x² - 2*w*x² + 4*x³
Step-by-step explanation:
The volume of any box is:
a * b * h       where a is the length, b is the wide and h is the height
Assuming we have cardboard of dimensions: l * w 
l - 2*x  will be length  of the box and   
w - 2*x will be wide of the box
h  =  x the height, then:
V(x) = ( l  -  2*x  ) *  (  w  -  2*x ) * x
V(x) = [ ( l*w - 2*l*x - 2*w*x + 4*x² ] * x
V(x) = l*w*x  - 2*l*x² - 2*w*x² + 4*x³