Change in enthalpy for reaction, $2{\mathrm{H}}_2{\mathrm{O}}_{2\left(\mathrm{l}\right)}\to 2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}$ if heat of formation of ${\mathrm{H}}_2{\mathrm{O}}_{2\left(\mathrm{l}\right)}$ and ${\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$ are -188 and – 286 kJ / mol respectively, is 

${{\mathrm{\Delta H}}^{\circ }}_{\mathrm{f}\left(\mathrm{products}\right)}-\sum {{\mathrm{H}}^{\circ }}_{\mathrm{f}\left(\text{substances}\right)}$
For the given reaction, 
$2{\mathrm{H}}_2{\mathrm{O}}_{2\left(\mathrm{l}\right)}\to 2{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}$
${{\mathrm{\Delta H}}^{\circ }}_{\mathrm{f}}=2\times {{\mathrm{\Delta H}}^{\circ }}_{\mathrm{f}\left({\mathrm{H}}_2\mathrm{O}\right)}-2\times {{\mathrm{\Delta H}}^{\circ }}_{\mathrm{f}\left({\mathrm{H}}_2{\mathrm{O}}_2\right)}$
$\begin{array}{l}=2\times -286{\mathrm{kJmol}}^{-1}-2\times (-188)\mathrm{kJ}{\mathrm{mol}}^{-1}\\ =-196\mathrm{kJ}{\mathrm{mol}}^{-1}\end{array}$