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k in (-oo:+oo)
3*k+3/7 = 15/7 // - 15/7
3*k+3/7-(15/7) = 0
3*k+3/7-15/7 = 0
3*k-12/7 = 0 // + 12/7
3*k = 12/7 // : 3
k = 12/7/3
k = 4/7
k = 4/7
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