The answer is 6.

Because in each row the circles corresponding to the centre column is always equal to half of the sum of the remaining circles in the same row ie.,

(4 + 2)/2 = 6 /2 = 3

(5+3+1+1)/2 = 10/2 = 5

6+1+2+3+3+1)/2 = 8

(7+2+4+3)/2 = 8

Therefore

(9+3)/2 = 6.