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k in (-oo:+oo)
2*(3-k) = 12 // - 12
2*(3-k)-12 = 0
2*(3-k)-12 = 0
-2*k-6 = 0
-2*k-6 = 0
-2*k-6 = 0 // + 6
-2*k = 6 // : -2
k = 6/(-2)
k = -3
k = -3
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