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k in (-oo:+oo)
(-4/7)*k+4 = 7*k+115/42 // - 7*k+115/42
(-4/7)*k-(7*k)-(115/42)+4 = 0
(-4/7)*k-7*k-115/42+4 = 0
53/42-53/7*k = 0 // - 53/42
-53/7*k = -53/42 // : -53/7
k = -53/42/(-53/7)
k = 1/6
k = 1/6
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