David's vehicle has a 21 gallon gas tank. If he drives only on highways, he can go about 525
miles on one tank of gas. If he drives only in the city, he can go about 378 miles on one tank of
gas. David drove 455 miles on his last tank of gas. If x is the amount of gallons used on highway
driving and y is the gallons used on city driving, which of the following best models the
relationship between 3 and y in this situation?