Suppose that every car on the road burned up about two gallons of gas a day. for each gallon of gas burned, one pound of carbon dioxide is released into earth’s atmosphere. if 144 million cars are used in the united states each day, how many pounds of carbon dioxide are released into the air each week?

You want to find pounds of CO2 / week = lb CO2 / week

1 lb CO2        2 gal gas        144 * 10^6 car          7day
x  x        x   
1 gal gas              car                   day                    week

Check what happen with units

 lbCO2      gal      car      day
x x x = lb CO2 / week
   gal         car      day    week

So the result is 2 * 144 * 10^6 * 7 lb CO2  / week = 2,016 - 10^6 lb CO2 / week

2,016 millions of pounds of CO2 per week.