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k in (-oo:+oo)
(3/7)*k+3/5 = (6/7)*k-(39/35) // - (6/7)*k-(39/35)
(3/7)*k-((6/7)*k)+3/5+39/35 = 0
(3/7)*k+(-6/7)*k+3/5+39/35 = 0
12/7-3/7*k = 0 // - 12/7
-3/7*k = -12/7 // : -3/7
k = -12/7/(-3/7)
k = 4
k = 4
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