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x in (-oo:+oo)
(2/3)*x = 8/21 // - 8/21
(2/3)*x-(8/21) = 0
(2/3)*x-8/21 = 0
2/3*x-8/21 = 0 // + 8/21
2/3*x = 8/21 // : 2/3
x = 8/21/2/3
x = 4/7
x = 4/7
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