12. (15 points) Fill in the grid with numbers 1 to 9. Each cell should contain one integer, and different cells should contain different numbers,

and the sum of the numbers in the cells surrounding each cell (i.e., cells that share a common edge with the cell) should be an integer multiple of the number in that cell. It is known that the cells to the left and right are already filled with the numbers 4 and 5, respectively. Question: What is the largest number that can be filled in the cell marked with the letter $x$?
【Analysis】According to the problem, among the numbers from 1 to 9, excluding 4 and 5, there are only 7 numbers left. We can first find the sum of these 7 numbers, which is 36. The middle $x$ can only be $3, 6, 9$. Therefore, we can test each one to determine the value of $x$.

【Solution】According to the analysis, $1+2+3+6+7+8+9=36$. The $x$ to be filled in must be a factor of the other five numbers, so $x$ can only be 3, 6, or 9. If $x=9$, then it is impossible for the sum of the surrounding numbers of each number to be an integer multiple of the number in that cell; when $x=6$, as shown in the figure, it is easy to see that $x=6$ meets the requirements.

Therefore, the answer is: 6.