Answer
-29.4 kJ
Work Step by Step
We obtain the required reaction by reversing the first reaction and then adding three times the second reaction with it.
Therefore, $\Delta G^{\circ}_{rxn}=-[\Delta G^{\circ}_{rxn}(\text{first reaction})]+3[\Delta G^{\circ}_{rxn}(\text{second reaction})]$
$=-(-742.2\,kJ)+3(-257.2\,kJ)=-29.4\,kJ$
