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k in (-oo:+oo)
(6/7)*k-((5/7)*k)+6 = 208/35 // - 208/35
(6/7)*k-((5/7)*k)-(208/35)+6 = 0
(6/7)*k+(-5/7)*k-208/35+6 = 0
1/7*k+2/35 = 0 // - 2/35
1/7*k = -2/35 // : 1/7
k = -2/35/1/7
k = -2/5
k = -2/5
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