Cathode Anode (Ag+(aq)+e−Mg→Ag)×2→Mg2+(aq)+2e− 
By Nernst equation $\begin{aligned}
& \mathrm{E}{\text {cell }}=\mathrm{E}{\mathrm{cell}}^{\circ}-\frac{\mathrm{RT}}{\mathrm{nF}} \ln \mathrm{Q} \
& \mathrm{E}{\text {cell }}=\mathrm{E}{\mathrm{cell}}^{\circ}-\frac{\mathrm{RT}}{\mathrm{nF}} \ln \frac{\left[\mathrm{Mg}^{2+}\right]}{\left[\mathrm{Ag}^{+}\right]^2} \
& =\mathrm{E}_{\text {cell }}^{\circ}+\frac{\mathrm{RT}}{2 \mathrm{~F}} \ln \frac{\left[\mathrm{Ag}^{+}\right]^2}{\left[\mathrm{Mg}^{2+}\right]}
\end{aligned}$