Determining the Average Bond Enthalpy for the C-F bond

Given the following data, how would you work out the average bond enthalpy for $\ce{C-F}$ bond. I've tried setting up the chemical equations and applying Hess's Law, but that's not getting me anywhere. 


$\Delta H\_\mathrm f^\circ(\ce{CF4(g)})=-680~\mathrm{kJ~mol^{-1}}$


Bond enthalpy, $\ce{F2(g)}=+158~\mathrm{kJ~mol^{-1}}$


$$\ce{C(s) -> C(g)}\quad \Delta H=+715~\mathrm{kJ~mol^{-1}}$$


EDIT: These are the equations I used:


$$\begin{align}
\ce{C(s) + 2F2(g) &-> CF4(g)}\\[6pt]
\ce{F2(g) &-> 2F-(g)}\\[6pt]
\ce{C(s) &-> C(g)}
\end{align}$$




Your approach to use Hess's Law is reasonable!


\[\Delta H\_r = -680 - (715 + 2\cdot158) = -1711\ \mathrm{kJ\cdot mol^{-1}}\]


That's the enthalpy for $\ce{CF4}$ - a molecule with **four** $\ce{C-F}$ bonds. 


The average $\ce{C-F}$ bond enthalpy is smaller:


\[\frac{1711}{4}\ \mathrm{kJ\cdot mol^{-1}} \approx 427\ \mathrm{kJ\cdot mol^{-1}}\]


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