Mathematical Models (PreCalc)

These are a few review problems I was given for a upcoming test and I'm completely stumped how to do these.

My main problem is the concept of "Expressing :blank: as a function of :blank:".

You can answer the problems completely if you'd like ( learning for my self would benefit me more but I could always do other problems ) but I just need a very dumbed down explanation.

Example 1)

The demand for a certain commodity is given $\displaystyle D(x) = -50x + 800; $ that is, x units of the commodity well be demanded by consumers when the price is $\displaystyle p = D(x) $ dollars pre unit. Total consumer expenditure $\displaystyle E(x) $ is the amount of money consumers pay to buy x units of the commodity.

A: Express consumer expenditure as a function of x, and sketch the graph of $\displaystyle E(x) $.

B: Use the in part A to determine the level of production x at which consumer expenditure is largest. What price $\displaystyle p $ corresponds to maximum consumer expenditure?

Example 2)

A spherical cell of radius r has volume $\displaystyle V = 4/3 pi r$

and surface are $\displaystyle S = 4 pi r^2$.

Express V as a function of S. If S is doubled, what happens to V?

Thanks for any help any one gives.