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k in (-oo:+oo)
(4/7)*k+4/7 = k-8/7 // - k-8/7
(4/7)*k-k+4/7-(-8/7) = 0
(4/7)*k-k+4/7+8/7 = 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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