V(X)=x^3-5x^2-8x+12

the depth is x+2
Find linear expressions with integer coefficients for the other dimensions and next if the depth of the chest is 9feet deep what are the other dimensions?

V = depth * width * length

V = (x+2) * w * l = x^3 - 5x^2 -8x +12
so divide x^3 - 5x^2 -8x +12 by (x+2)
and get
l * b = x^2 -7x +6
which is
(x-6)(x-1)
so
length = x-1
width = x-6
depth = x+2
take it from there