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k in (-oo:+oo)
3*(1-6*k) = -213 // + 213
3*(1-6*k)+213 = 0
3*(1-6*k)+213 = 0
216-18*k = 0
216-18*k = 0
216-18*k = 0 // - 216
-18*k = -216 // : -18
k = -216/(-18)
k = 12
k = 12
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