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k in (-oo:+oo)
(-2/7)*k-6 = (6/7)*k-50/7 // - (6/7)*k-50/7
(-2/7)*k-((6/7)*k)-(-50/7)-6 = 0
(-2/7)*k+(-6/7)*k+50/7-6 = 0
8/7-8/7*k = 0 // - 8/7
-8/7*k = -8/7 // : -8/7
k = -8/7/(-8/7)
k = 1
k = 1
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