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m in (-oo:+oo)
(5/7)*m = 5/21 // - 5/21
(5/7)*m-(5/21) = 0
(5/7)*m-5/21 = 0
5/7*m-5/21 = 0 // + 5/21
5/7*m = 5/21 // : 5/7
m = 5/21/5/7
m = 1/3
m = 1/3
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