y = mx + b

m is the slope, or change in y/change in x, dollard per month for example.

b is the y-intercept. If x = 0 (no months), how much are we paying?

If x = the number of months, we are paying $15x for however many months. After 7 months, we've paid 15(7) = $105.

But wait, there was a one time fee of $20, so in 7 months we've paid 105 + 20 = $125.

So our equation would look like...

**y = 15x + 20**

y = 15(12) + 20

y = 180 + 20

y = 200

After one year, we've paid $200.

b = 20, because before we pay for any months (at 0 months), our charge is $20.

m = 15, because we are paying $15/month.