Healthy Shaoxing a

One-to-one correspondence of elements, 1= 1,

So 4=x? Or 2x=x? ：

4=x？ , x=2 or x=-2, but when x=2, 4=2x, so x=2 is discarded, leaving x =-2;

2x=x？ , x=0 or x= 1, but when x= 1, x? = 1, so x= 1 is discarded, leaving x=0.

To sum up, x=0 or x=-2.