A certain car gets 25 miles per gallon when the tires are properly inflated. For every pound of pressure that the tires are underinflated, the gas mileage decreases by 12 mile per gallon. Find the equation that relates miles per gallon, y, to the amount that the tires are underinflated, x. Use the equation to calculate the gas mileage when the tires are underinflated by 8 pounds of pressure.