If you would like to solve the system of equations x + y = 3 and 2x - y = 6, you can do this using the following steps:
x + y = 3 ... y = 3 - x
2x - y = 6
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2x - y = 6
2x - (3 - x) = 6
2x - 3 + x = 6
3x = 6 + 3
3x = 9 /3
x = 9/3
x = 3
y = 3 - x = 3 - 3 = 0
The correct result would be x = 3.