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n in (-oo:+oo)
6*n-(3*n)+3 = 30 // - 30
6*n-(3*n)-30+3 = 0
6*n-3*n-30+3 = 0
3*n-27 = 0 // + 27
3*n = 27 // : 3
n = 27/3
n = 9
n = 9
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