You've got 11 units of stuff and you need to parcel them out among a, b, and c. This is a combinations with repetition problem, with 3 types (2 dividers) and 11 objects. So there are $${ {11+2} \choose 2 } = {{13} \choose 2}$$ ways to do this.
The answer \({13} \choose 11\) is equivalent.