The current number of coupons being mailed out by a company each week in one region of the country can be represented by the function R(x) = 3x2 + 21x, where x is the number of cities in the region. The number of coupons in a new mailing sent only to preferred customers can be represented by the function P(x) = 9x. Which function best describes C(x), the number of coupons that are shipped out in any given week?