If base is r and you need to find r's complement then first find (r-1)'s complement.
Follow these steps:-
1. Subtract each digit of the given number from (r-1).
Here r=9 so r-1=8. Subtract 247 from 888 to get 641.
2. Then add 1 to it. 641+1=642
You can verify it for binary numbers, r=2. Say N=1011 then 1's complement is 0100. How we get it? By subtracting each digit drop from 1. So 1111-1011=0100. And add 1 so 2's complement becomes 0101.