Add '23.6' on the RHS nad the LHS of the above expression.
[tex]\begin{gathered} m-23.6+23.6=58.3+23.6 \\ m-0=81.9 \\ m=81.9 \end{gathered}[/tex]Thus, the value of m is 81.9. And the corect option is, option (A) 81.9, because to isolate m, you would add 23.6 and 58.3.