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

Start by distributing the 15 to the (m-1). 9+12m=6+15m-15. Then combine like terms. 9+12m=15m-9. Next move the variable all to one side, it doesn't matter which variable you choose. I'll use 12m so when I subtract I am still left with a postive coefficient. To move the 12m we subtract it from both sides of the equation. We are left with 9=3m-9. Next we isolate the m by canceling out the 9. We do this by adding 9 to both sides. We are left with 18=3m. Finally, to completely isolate the m, we must divide both sides by the coefficient, 3. The result is 6=m. The answer is 6 months. You can check this by plugging 6 back into the equation where there is an m. (sorry for the long answer)