899$ is the y intercept because that is your starting value. Then, 65 is your slope. (Rise 65, run 1). After 24 months, the total cost will be 2459$. This is because 24 • 65 = 1560. This is your total cost of phone service. Then, you need to add the phone. 1560 + 899 = 2459. So you total cost is 2459.
