A box with a volume of $\displaystyle 400 cm^{3} $has the shape of a rectangular prism. It has a fixed height of $\displaystyle 25 cm$, a length of y cm and a width of x cm. If $\displaystyle A cm^{2}$ is the total surface area:

a) Express A in terms of x.

I don't understand what it means when it says 'express A in terms of x'. Anyway if someone could provide detailed working out to this it will be appreciated!