It's an odd question, because it looks to me that there is more than one correct answer.
First of all, they must mean "...so that it shows a function of x" because it's already not a function of y -- since there are two x values when y = 8.
Now, since there are already rows for x values 6, 3, 9, and 7 you can't use any of those numbers for x. That's because a function can only have one y value for each x value, so you can't repeat the same x value.
So either 8 or 12 for x should work, and then any y value can be used.
But maybe I'm not understanding the question?