If the container is divided into 12ths you should make that your common denominator.
So 1/4 now equals 3/12
Since we want this 3/12 to be empty we must subtract it from 12/12 leaving us with 9/12.
When we subtract 2/12 from 9/12 to get the amount that was already filled we get 7/12 which is the amount that needs to be filled.
To check we add our parts 2/12 and 7/12 to get 9/12 which is 3/4 of the container filled leaving 3/12 or 1/4 of the container empty.
Hope this helps ☺