To find the standard potential of $M^{3+}/M$ electrodes the following cell is constituted. $\mathrm{Pt} M/M^{3+}(0.001\mathrm{mol}{\mathrm{L}}^{-1})/{\mathrm{Ag}}^{+}(0.01\mathrm{mol}{\mathrm{L}}^{-1})/\mathrm{Ag}$ the emf of the cell is found to be 0.421 volt at 298K. the standard potential of half reaction $M^{3+}+3e^{-}\to M$  at 298K will be $Given{E}_{A{g}^{+}/Ag}^{0}at298K=0.80Volt$ )