$\Delta {\text{H}}{\mkern 1mu} {\mkern 1mu} = {\mkern 1mu} {\mkern 1mu} \sum {{{({H_f}^\circ )}_{\Pr oduct}}} {\mkern 1mu} {\text{ - }}{\mkern 1mu} \sum {{{{\text{(}}{{\text{H}}_f}^\circ {\text{)}}}_{{\text{Reactant}}}}} {\text{ }}$
$\Delta {{\text{H}}^ \circ }\, = \,\,\{ ( - 393.5)\,\, + \,2( - 286)\} \,\,\,\,\,\, - \,\{ ( - 75)\,\, + \,\,2(0)\} $
$\Delta {H^ \circ }\, = \,\,( - 965.5)\,\, - \,\,( - 75)\,\, = \,\, - \,\,890.5\,\,kJ$