tolong dibantu jawab yang benar kak, aku kasih 30 poin

jawab
a - b = 8 
b + c = 1 || (+)
a + c = 9

2a + c = 7
a  + c = 9 || (-)
a = - 2

a - b = 8
-2 - b = 8
b = -10

a + c = 9
-2 + c = 9
c = 11

a - b = 8 .....................(1)
c + 2a = 7 ..................(2)
b + c = 1 ....................(3)

Dari (1) dan (3)
a - b       = 8
     b + c = 1     +
----------------------
a + c = 9 ..................(4)


Dari (2) dan (4)
2a + c = 7
a + c = 9    -
----------------
a = -2

subtitusi nilai a = -2 ke pers a + c = 9
a + c = 9
-2 + c =9
c = 9 + 2
c = 11

subtitusikan nilai a = -2 ke pers (1)
a - b = 8
-2 - b = 8
b = -10

jadi 
a = -2
b = -10
c = 11