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k in (-oo:+oo)
7*(3*k+6) = 252 // - 252
7*(3*k+6)-252 = 0
7*(3*k+6)-252 = 0
21*k-210 = 0
21*k-210 = 0
21*k-210 = 0 // + 210
21*k = 210 // : 21
k = 210/21
k = 10
k = 10
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