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.