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X in (-oo:+oo)
X/39 = 4/12 // - 4/12
X/39-(4/12) = 0
X/39-1/3 = 0
1/39*X-1/3 = 0 // + 1/3
1/39*X = 1/3 // : 1/39
X = 1/3/1/39
X = 13
X = 13
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