Oxidation Reduction Potentials: Nernst's Law

Here are two oxidation-reduction systems with their standard →         potentials : $O_2(g)+4H^+(aq)+4e^- $ $\leftrightarrows$ $2H_2O(l)$     $E^o_1$= $1.23 \;V$ $Ag^{+}(aq)+e^- $ $\leftrightarrows$ $Ag(s)$    $E^o_2$= $0.80 \;V$ Calculate the equilibrium constant at $ 25^oC $ (standard!) of the reaction $4Ag(s)$ -4$e^-+O_2(g)+4H^+(aq)$ +4$e^-$ $\leftrightarrows$ $4Ag^+(aq)+2H_2O(l)$