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k in (-oo:+oo)
2/14 = k/21 // - k/21
2/14-(k/21) = 0
1/7-1/21*k = 0 // - 1/7
-1/21*k = -1/7 // : -1/21
k = -1/7/(-1/21)
k = 3
k = 3
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