[(m²-9)分之(21-5m) -(m²-9)分之m]÷ [(m+3)分之m -(m+3)分之(m-3)]
=[(m²-9)分之(21-5m-m)]÷ [(m+3)分之3]
=[(m²-9)分之(21-6m)]× 3分之(m+3)
=(m-3)分之(7-2m)
=-2 + (m-3)分之1
=-2 + (-6分之1 -3)分之1
=-2 - (6分之19)分之1
=-2 - 19分之6
=-2又19分之6
当m=-6分之1时,代数式m²-9分之21-5m-m²-9分之m除于m+3分之m-m+3分之m-3的