is a 1 followed by 36 zeros. To subtract the 63 we have to borrow from the 1 (so it will be gone after subtracting). Then 34 of the 36 zeros will be 9's and the last two digits will be 3 and 7:
1000000000000000000000000000000000000
- 63
_______________________________________
999999999999999999999999999999999937
So the sum of the digits would be 34*9+3+7 = 306+3+7 = 316.