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