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k in (-oo:+oo)
-214/35 = (3/7)*k-k-6 // - (3/7)*k-k-6
k-((3/7)*k)-214/35+6 = 0
k+(-3/7)*k-214/35+6 = 0
4/7*k-4/35 = 0 // + 4/35
4/7*k = 4/35 // : 4/7
k = 4/35/4/7
k = 1/5
k = 1/5
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