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m in (-oo:+oo)
5*(4-2*m) = -20 // + 20
5*(4-2*m)+20 = 0
5*(4-2*m)+20 = 0
40-10*m = 0
40-10*m = 0
40-10*m = 0 // - 40
-10*m = -40 // : -10
m = -40/(-10)
m = 4
m = 4
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