Thermodynamics

If the standard enthalpy change $\left(\Delta_r H^{\circ}\right)$ for reaction $\mathrm{H}_2(\mathrm{~g})+\mathrm{Br}_2(l) \longrightarrow 2 \mathrm{HBr}(\mathrm{g})$ is -72.8 kJ , the standard enthalpy of formation $\left(\Delta_f H^{\circ}\right)$ of $\mathrm{HBr}(g)$ (in $\mathrm{kJ} \mathrm{mol}^{-1}$ ) is