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m in (-oo:+oo)
8/6 = m/21 // - m/21
8/6-(m/21) = 0
4/3-1/21*m = 0 // - 4/3
-1/21*m = -4/3 // : -1/21
m = -4/3/(-1/21)
m = 28
m = 28
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