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n in (-oo:+oo)
6*n+3 = 3*n+39 // - 3*n+39
6*n-(3*n)-39+3 = 0
6*n-3*n-39+3 = 0
3*n-36 = 0 // + 36
3*n = 36 // : 3
n = 36/3
n = 12
n = 12
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