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k in (-oo:+oo)
8-6*k-(7*k) = -31 // + 31
8-6*k-(7*k)+31 = 0
8-6*k-7*k+31 = 0
39-13*k = 0 // - 39
-13*k = -39 // : -13
k = -39/(-13)
k = 3
k = 3
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