21.
4 gallons of gasExplanation:Since y = 86. y+6 = 23x. Then the equation becomes 86+6 = 23 x. When you solve this you will get x = 4. In other words, this car consumes 4 gallons of gas to 86 miles away. In other words, this car's gas (fuel) efficiency is 4.65 gallons of gas per one hundred mile.
