In an endothermic reaction, the value of $∆\mathrm{H}$ is:

From the given reactions
$\mathrm{S} \left(\mathrm{s}\right)+\frac{{\displaystyle 3}}{{\displaystyle 2}}{\mathrm{O}}_2 \left(\mathrm{g}\right)⟶{\mathrm{SO}}_3 \left(\mathrm{g}\right)+2\mathrm{x} \mathrm{kcal}$

and ${\mathrm{SO}}_2 \left(\mathrm{g}\right)+\frac{{\displaystyle 1}}{{\displaystyle 2}}{\mathrm{O}}_2 \left(\mathrm{g}\right)⟶{\mathrm{SO}}_3 \left(\mathrm{g}\right)+\mathrm{y} \mathrm{kcal}$

the heat of formation of ${\mathrm{SO}}_2$ is:

Change in enthalpy for reaction
$2{\mathrm{H}}_2{\mathrm{O}}_2 \left(\ell \right)⟶2{\mathrm{H}}_2\mathrm{O} \left(\ell \right)+{\mathrm{O}}_2 \left(\mathrm{g}\right)$

If heat of formations of ${\mathrm{H}}_2{\mathrm{O}}_2\left(\ell \right)$ and ${\mathrm{H}}_2\mathrm{O}\left(\ell \right)$ are $-188$ and $-286\mathrm{kJ}/\mathrm{mol}$, respectively is: