Suppose the total cost of producing x units of a particular commodity is modeled by C(x) = 1/7x^2 + 4x + 70 and that each unit of the commodity can be sold for p(x) = 1/4(90 x) dollars. a. What is the marginal cost? (Use differentials to approximate your answer.) b. What is the price when the marginal cost is 10? c. Estimate the cost of producing the 11th unit. (Use differentials to approximate your answer. Round your answer to two decimal places.) d. Find the actual cost of producing the 11th unit.