What is this question asking

Remember, a number's additive inverse is simply its opposite.
Let's say we have a number a.
The opposite of a is -a, and the opposite of -a is a.
Thus, the additive inverse of
is

Hope it helps.
~Just a felicitous girl
#HaveAnAmazingDay
Feel free to ask if you have any doubts.

5/9 because you have to add those two together and put that number over the total number of marbles.
The number is 21 because 2 is 1 doubled, and 2+1 is 3.