AldrieeKnight: 2a+c=-10
b-c=15
a-2b+c=-5
How you do it, and not just the answer...
2a+c = -10 --> 2a = -10 - c --> a = -5 - c/2
b-c = 15 --> b = 15 + c
Use these in the final equation.
a - 2b + c = -5 --> - 5 - c/2 - 30 - 2c + c = -5 --> -35 - 3c/2 = -5 --> 3c/2 + 35 = 5 --> 3c/2 = -30 --> c = -20.
Now use this c in the top ones.
a = -5 +20/2 = -5 + 10 = 5
b = 15 + c = 15 - 20 = -5
