Burning one gallon of gasoline releases 20 pounds of co2 into the air. harry’s compact car can travel 25 miles on one gallon of gas. if harry drives 15,000 miles in a year, his vehicle will add pounds of co2 to the atmosphere.

There are several information's already given in the question. Based on those information's, the answer can be easily deduced.
Amount of gasoline required by Harry's car to travel 25 miles = 1 gallon
Then
amount of gasoline required
by Harry's car to travel 15000 miles = 15000/25
                                                         = 600 gallons
So 
Amount of CO2 released by burning 1 gallon of gasoline = 20 pounds
Then
Amount of CO2 released
by burning 600 gallon of gasoline = 600 * 20
                                                      = 12000 pounds
From the above deduction, it can be concluded that the amount of CO2 that will be added by Harry's car to the atmosphere is 12000 pounds.

Yes you are correct use the scientific method