Answer
$-847.6\,kJ/mol$
Work Step by Step
After reversing the third reaction and then adding it with the second reaction, we get the first reaction. Therefore, $\Delta H^{\circ}_{rxn}$ is equal to the sum of $\Delta H^{\circ}_{rxn}$ for the second reaction and the negative of $\Delta H^{\circ}_{rxn}$ for the third reaction.
That is, $\Delta H^{\circ}_{rxn}=-(-822.2\,kJ/mol)+(-1669.8\,kJ/mol)$
$=-847.6\,kJ/mol$
