The cost to fill a car's tank with gas and get a car wash is a linear function of the capacity of the tank. The costs of a fill-up and a car wash for three different customers are shown in the table. Write an equation for the function in slope-intercept form. Then, find the cost of a fill-up and a car wash for a customer with a truck whose tank size is 31 gallons.

1. f(x) = 1.90x + 4.25
Cost for truck = $63.15

2. f(x) = 1.80x + 3.75
Cost for truck = $59.55

3. f(x) = 2.00x + 4.25
Cost for truck = $66.25

4. f(x) = 0.56x + 2.08
Cost for truck = $19.44