The given cell notation is:
[tex]Mg(s)|Mg^{+2}(aq)||Ag^{+}(aq)|Ag(s)[/tex]
Thus here Mg is undergoing oxidation [as written on left side]
silver is undergoing reduction [ as written on right side]
In oxidation Magnesium will lose two electrons per atom
In reduction silver will gain one electron per atom
In order to balance the redox reaction we will multiply silver with two so that overall there will be exchange of two electrons
Oxidation half reaction:
[tex]Mg(s)--->Mg^{+2}(aq)+2e[/tex]
Reduction half reaction:
[tex]Ag^{+}(aq)+e-->Ag(s)[/tex]
Now we will multiply the reduction equation by two
Overall redox reaction:
[tex]Mg(s)+2Ag^{+}(aq)-->Mg^{+2}(aq)+2Ag(s)[/tex]