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m in (-oo:+oo)
(6/7)*m-(1/3) = -1/21 // + -1/21
(6/7)*m-(1/3)-(-1/21) = 0
(6/7)*m-1/3+1/21 = 0
6/7*m-2/7 = 0 // + 2/7
6/7*m = 2/7 // : 6/7
m = 2/7/6/7
m = 1/3
m = 1/3
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