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n in (-oo:+oo)
-12 = n-3*n-6 // - n-3*n-6
6-(-3*n)-n-12 = 0
3*n-n-12+6 = 0
2*n-6 = 0 // + 6
2*n = 6 // : 2
n = 6/2
n = 3
n = 3
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