A+B= 13,A+C=9,B+C= 12
So,
(A+B)+(A+C)+(B+C)= 13+9+ 12 = 34
that is
2A+2B+2C=34
A+B+C= 17
Therefore, a =17-(b+c) =17-12 = 5.
B= 17-(A+C)= 17-9=8
c = 17-(A+B)= 17- 13 = 4