Manufacturing—Sunglasses: The number of unbreakable sunglasses sold at the U-C-Me specialt...

Manufacturing—Sunglasses: The number of unbreakable sunglasses sold at the U-C-Me specialty store during a long-term sales promotion can be modeled by

where x represents the number of months since the promotion began, and f(x) represents the number of sunglasses sold. Determine the secant line slope and interpret the result.