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m in (-oo:+oo)
2*m-8 = 7*m-28 // - 7*m-28
2*m-(7*m)-8+28 = 0
2*m-7*m-8+28 = 0
20-5*m = 0 // - 20
-5*m = -20 // : -5
m = -20/(-5)
m = 4
m = 4
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