The equation 9+12m=6+15(m-1) models the following situation: Movie-rental company A charges a sign-up fee of $9 plus $12 per month, and company B charges $6 for the first month plus $15 per month after that. At a certain number of months, the cost of the two companies is equal. Which equation is a simplified version of the one above? 3m = 15 9 + 15m = 12m – 1 3m = 18 9 + 6 – 15 = 15m – 12m