3864 - (3 8 6 4) => => (0011 1110 1100 0100) is 2421 representation
9's complement of 3864 is 6135 which in 2421 representation is => (1100 0001 0011 1011)
(0011 1110 1100 0100) = (3864)10
(1100 0001 0011 1011) = (6135)10
Compare the two, we get each other by interchanging 0's and 1's
Self-complementing codes provide the 9’s complement of a decimal number, just by interchanging 1’s and 0’s in its equivalent 2421 representation.