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k in (-oo:+oo)
(1/3)*k-((5/7)*k)-1/2 = -31/126 // + -31/126
(1/3)*k-((5/7)*k)-1/2-(-31/126) = 0
(-5/7)*k+(1/3)*k-1/2+31/126 = 0
-8/21*k-16/63 = 0 // + 16/63
-8/21*k = 16/63 // : -8/21
k = 16/63/(-8/21)
k = -2/3
k = -2/3
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