`2678 : [462 - 2(x + 6)] = 13`
`462 - 2(x + 6) = 2678 : 13`
`462 - 2(x + 6) = 206`
`2(x + 6) = 462 - 206`
`2(x + 6) = 256`
`x + 6 = 256 : 2`
`x + 6 = 128`
`x = 128 - 6`
`x = 122`
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`2678 : [462 - 2(x - 6)] = 13`
`462 - 2(x - 6) = 2678 : 13`
`462 - 2(x - 6) = 206`
`2(x - 6) = 462 - 206`
`2(x - 6) = 256`
`x - 6 = 256 : 2`
`x - 6 = 128`
`x = 128 + 6`
`x = 134`