Calculate the standard enthalpy variation of the following reaction consulting the → Table of formation enthalpies : $2NH_3(g)$ $+$ $3O_2(g)$ $+$ $2CH_4(g)$ $\longrightarrow$ $2HCN(g)$ $+$ $6H_2O(g)$
$\Delta H^o$ $=$ $(6\Delta H_f(H_2O(g))$ $+$ $2\Delta H_f(HCN(g))$ $-$ $(2\Delta H_f(NH_3(g)$ $+$ $2\Delta H_f(CH_4(g))$ $=$ $-938.5 \;kJ$