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k in (-oo:+oo)
3*(6-k) = -3 // + 3
3*(6-k)+3 = 0
3*(6-k)+3 = 0
21-3*k = 0
21-3*k = 0
21-3*k = 0 // - 21
-3*k = -21 // : -3
k = -21/(-3)
k = 7
k = 7
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