A company has found that the marginal cost (in thousands of dollars) to produce x central air conditioning units is Upper C prime (x )equals StartFraction 18 x Over 3 x squared plus e EndFraction , where x is the number of units produced. (a) Find the cost function, given that the company incurs a fixed cost of $15 comma 000 even if no units are built. (b) The company will seek a new source of investment income if the cost is more than $21 comma 000 to produce 5 units. Should they seek this new source?