The unbalanced chemical reaction is:
Mn3+(aq)→Mn2+(aq)+MnO2(s)+H+(aq)
The oxidation half reaction is,
Mn3+(aq)→MnO2(s).
To balance oxidation number, one electron is added on R.H.S.
Mn3+(aq)→MnO2(s)+e−
4 protons are added to balance the charge.
Mn3+(aq)→MnO2(s)+4H+(aq)+e−
2 water molecules are added to balance O atoms.
The reduction half reaction is Mn3+(aq)→Mn2+(aq).
An electron is added to balance oxidation number.
Mn3+(aq)+e−→Mn2+(aq)
Two half cell reactions are added to obtain balanced chemical equation.
2Mn3+(aq)+2H2O(l)→Mn2+(aq)+MnO2(s)+4H+(aq)