You'll want to solve the whole equation to find out y, and this is what I got :)
This is a fun little question, and I found out the answer by discovering that the answer (-28) is a multiple of 7 (7 x 4) and went from there!
I'm not exactly sure if there are other answers for this question, or if this is the best one, but this is what I got when I supplemented both with 7!
3x - 7y = -28
3(7) - 7(7) = -28
21 - 49 = -28 !
If the numbers were switched in the equation, the outcome would be positive haha
Good luck and have a great day!