I just lost a very time consuming answer. But yes that number mod 15 is 0 so a must be zero.
Here is the short version.
This is a very weirdly presented question. That number is base 74 (very strange) but 15 is base 10
Use the fact that 74 mod 15 = -1
So when mod 15 of that number is taken we get
2-3+5-9+3-5+6-2+3 = 0
In other words that number is already a multiple of 15 (base10) so a = 0