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x in (-oo:+oo)
26/21 = x+4/7 // - x+4/7
26/21-x-(4/7) = 0
26/21-x-4/7 = 0
2/3-x = 0 // - 2/3
-x = -2/3 // * -1
x = 2/3
x = 2/3
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