The manager of an electronics store estimates that the number of flash drives that they will sell at a price of x dollars is S(x) = 2,100 x + 9 Find the rate of change of this quantity when the price is $4 per flash drive, and interpret your answer. (Round your answer to the nearest whole number.) The number of flash drives sold at $4 each is increasing X by about -12 in price. × flash drives per dollar increase