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m in (-oo:+oo)
2*(m+3) = (1/3)*(15*m+21) // - (1/3)*(15*m+21)
2*(m+3)-((1/3)*(15*m+21)) = 0
2*(m+3)+(-1/3)*(15*m+21) = 0
2*(m+3)-1/3*(15*m+21) = 0
2*(m+3)-1/3*(15*m+21) = 0
-3*m-1 = 0
-3*m-1 = 0
-3*m-1 = 0 // + 1
-3*m = 1 // : -3
m = 1/(-3)
m = -1/3
m = -1/3
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