Bond Enthalpy Questions

The standard enthalpy of formation of ${\text{NH}}_3$ is $-46.0{\text{kJ mol}}^{-1}$. If the enthalpy of formation of ${\text{H}}_2$ from its atoms is $-436{\text{kJ mol}}^{-1}$ and that of ${\text{N}}_2$ is $-712{\text{kJ mol}}^{-1}$, the average bond enthalpy of $\text{N}-\text{H}$ bond in ${\text{NH}}_3$ is: