х + 2у = 12 | *3
2х - 3у = -18 | *2
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3х + 6у = 36
4х -6у = -36
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3х + 4х + 6у -6у = 36 + (-36)
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3х + 4х + 6у -6у = 36 -36
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3х + 4х = 0
х = 0
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0 + 2у = 12
2у = 12
у = 6
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х = 0
у = 6