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k in (-oo:+oo)
3/8-2*k-((2/7)*k) = 405/56 // - 405/56
3/8-2*k-((2/7)*k)-(405/56) = 0
(-2/7)*k-2*k+3/8-405/56 = 0
-16/7*k-48/7 = 0 // + 48/7
-16/7*k = 48/7 // : -16/7
k = 48/7/(-16/7)
k = -3
k = -3
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