So I'm having troubles with this question...

A small open box for blueberries is to be made from a rectangular piece of cardboard, 15 cm by 9 cm, by cutting equal suares from the corners and turning up the sides. Let x represent the length of the sides of the squares removed.

a). Write an equation for the volume of the box as a funtion ofx, V(x). Specifiy theappropirate domain of this polynomial function.

b). Find the exact values of x such that the volume is 56 cm. Which of these values is a physical impossibility in the construction of the box? Which value of x is most appropirate in the problem context?

So I know that Volume is length x width x height... but what do I do..

EDIT: So I found the answer from part A which is 4x^3-48x^2+135x

How would I do question b?

Thanks!