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k in (-oo:+oo)
12 = k+2*k-9 // - k+2*k-9
9-k-(2*k)+12 = 0
9-k-2*k+12 = 0
21-3*k = 0 // - 21
-3*k = -21 // : -3
k = -21/(-3)
k = 7
k = 7
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