I have homework problems, but I have no idea where to begin. There is no other background information.

One liter of air at sea level contains _______________ molecules. This would be for 25°C and sea level pressure 1 atm, not "STP".
You can find the concentration of air molecules at sea level in the videos or from your first discussion activity, in molecules per cubic centimeter; note that one liter contains 1000 cubic centimeters

How many molecules do 2 liters of air contain at 11 km height? _______________
Note that air concentration is like a density; look in the videos for the way the density changes with height, particularly how much it changes for every 5.5 km increase in elevation