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m in (-oo:+oo)
2*m-(6*m)-4 = 20 // - 20
2*m-(6*m)-20-4 = 0
2*m-6*m-20-4 = 0
-4*m-24 = 0 // + 24
-4*m = 24 // : -4
m = 24/(-4)
m = -6
m = -6
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