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k in (-oo:+oo)
51 = k+6*k+9 // - k+6*k+9
51-k-(6*k)-9 = 0
51-k-6*k-9 = 0
42-7*k = 0 // - 42
-7*k = -42 // : -7
k = -42/(-7)
k = 6
k = 6
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