Answer
$ M=M_0\cdot 10^{\frac{6-m}{2.5}} $
Work Step by Step
$m=6-2.5\ log(\frac{M}{M_0})\longrightarrow log(\frac{M}{M_0})=\frac{6-m}{2.5} \longrightarrow \frac{M}{M_0}=10^{\frac{6-m}{2.5}} \longrightarrow M=M_0\cdot 10^{\frac{6-m}{2.5}} $
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