I'm not exactly sure of the question you're asking, but I'll assume it's "where does the marginal revenue equal the marginal cost?"

The marginal revenue and marginal cost functions are the derivatives of the total revenue and total cost functions. So

MC = 3

MR = 7-2X

And X is solved by setting 3 = 7-2X, since you want to produce where the MC = MR. In this case, that works out to X=2, which, if I understand your note below, implies you would produce 2,000 units.

- Steve J