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