Respuesta :

You didn't put a + charge on one of the Ag. It has to be on the left side 

2H2O + MnO2 ==> MnO4^- + 3e + 4H^+ 

Ag^+ + e ==> Ag^+ 
Those are the two BALANCED half reactions. Multiply equation 1 by 1 and equation 2 by 3 and add them.

Answer:

2H2O + MnO2 + 3Ag^+-------------> MnO4^- + 4H^+ + 3Ag

Explanation:

Oxidation half equation:

2H2O + MnO2 -------------> MnO4^- + 3e + 4H^+

Reduction half equation:

3Ag^+ + 3e -------------> 3Ag

Overall redox reaction equation:

2H2O + MnO2 + 3Ag^+-------------> MnO4^- + 4H^+ + 3Ag

The hydrogen ions comes from water.