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