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`1/2 a+6 = 13-a || +a `

`3/2 a+6 = 13 || -6 `

`3/2a = 7 || *2/3 `

`a=14/3 `

Probe:

`1/2 * 14/3 + 6 = 13 - 14/3 `

`7/3 + 18/3 = 39/3 - 14/3 `

`25/3 = 25/3 `✔✔

`=> L={14/3} `