You may want to double check me, but I believe it works like this:

Marginal Revenue is derived by taking the derivative of the total revenue.

Total revenue = price per unit * quantity sold.

So in this case, total revenue = (-7X+2000) * X = -7X^2 + 2000X

Marginal revenue is the first derivative with respect to X, or -14X + 2000.

Does that make sense?

- Steve J