A + b - c = 4 -a + 2b + 2c = -2 B - 2c = 7 - 2a Nilai 2c = ...

a + b - c = 4 ...pers.1
-a + 2b + 2c = -2 ...pers.2
b - 2c = 7 - 2a → 2a + b - 2c = 7 ...pers.3

Eliminasi pers.1 dan pers.2
a + b - c = 4
-a + 2b + 2c = -2
______________+
3b + c = 2 ...pers.4

Eliminasi pers.2 dan pers.3
-a + 2b + 2c = -2 |× 2|→ -2a + 4b + 4c = -4
2a + b - 2c = 7    |× 1|→ 2a + b - 2c = 7
                                      ________________+
                                        5b + 2c = 3 ...pers.5

Eliminasi pers.4 dan pers.5
3b + c = 2   |× 2|→ 6b + 2c = 4
5b + 2c = 3 |× 1|→ 5b + 2c = 3
                             _____________-
                               b = 1
3b + c = 2
3 × 1 + c = 2
3 + c = 2
c = 2 - 3
c = -1

a + b - c = 4
a + 1 - (-1) = 4
a + 1 + 1 = 4
a + 2 = 4
a = 4 - 2
a = 2

Nilai 2c = 2 × (-1) = -2

Semoga membantu kalau ngak salah begini hehe