A machine parts company collects data on demand for its parts. If the price is set at $44.00, then the company can sell 1000 machine parts. If the price is set at $40.00, then the company can sell 1500 machine parts. Assuming the price curve is linear, construct the revenue function as a function of a items sold. R(x) = Find the marginal revenue at 400 machine parts. MR(400)=