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m in (-oo:+oo)
6/7 = m/21 // - m/21
6/7-(m/21) = 0
6/7-1/21*m = 0 // - 6/7
-1/21*m = -6/7 // : -1/21
m = -6/7/(-1/21)
m = 18
m = 18
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