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N in (-oo:+oo)
12/28 = N/21 // - N/21
12/28-(N/21) = 0
3/7-1/21*N = 0 // - 3/7
-1/21*N = -3/7 // : -1/21
N = -3/7/(-1/21)
N = 9
N = 9
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