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m in (-oo:+oo)
3/7-3*m = -51/28 // + -51/28
3/7-3*m-(-51/28) = 0
3/7-3*m+51/28 = 0
9/4-3*m = 0 // - 9/4
-3*m = -9/4 // : -3
m = -9/4/(-3)
m = 3/4
m = 3/4
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