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 per year, his vehicle will add pounds of co2 to the atmosphere.

It was given that

1 gal = 20 lbs of CO2
1 gal = 25 miles distance

Thus, 

15000 miles/year ( 1 gal / 25 miles ) ( 20 lbs CO2 / 1 gal ) = 12000 CO2

If Harry drives 15,000 miles per year, his vehicle will add 12000 pounds of CO2 to the atmosphere.

The graph isn’t that clear

Where’s the graph too show me?