In each column, the bottom digit is the sum of the top two digits plus 1, mod 10; in other words, that sum's units digit:

5+2+1=8
3+6+1=10 = 0 mod 10
7+4+1=12 = 2 mod 10
8+1+1=10 = 0 mod 10

then

9+4+1=14 = 4 mod 10

so the middle 4 in this sum should be the x.