I’m very new to chemistry so if you can help me that would be great!

The answer would be 21. I got this answer because the subscript numbers that are to the right of each element are the amounts of atoms. So there is 6 carbon atoms, 12 hydrogen atoms, 2 nitrogen atoms, and 1 oxygen atom in one molecule of the chemical formula you provided me with. But since the question asks us for two of these molecules, just add up the amount of atoms in one molecule and multiply by 2.

D)can be increased in a closed system