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m in (-oo:+oo)
(1/3)*m+4/7 = -23/21 // + -23/21
(1/3)*m+4/7-(-23/21) = 0
(1/3)*m+4/7+23/21 = 0
1/3*m+5/3 = 0 // - 5/3
1/3*m = -5/3 // : 1/3
m = -5/3/1/3
m = -5
m = -5
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