the 6971 is the original number you started with, you are now decreasing by 7, so you use the value of -7. The x is the number of years that pass by, so the variable x affects the number 7 because it says decreases by 7 per year. The x does not go with 6971 as that number doesn't get directly affected.
If you know the whole price for 2.4 gallons, divide the whole price by 2.4 to get the cost for one, because when you multiply the cost for 1 by 2.4, you get the total.
