Given the following Gibbs energies at $25^{\circ} \mathrm{C}$, calculate the solubility-product constant of (a) $\mathrm{AgCl}(s)$ and (b) $\operatorname{AgBr}(s)$
\begin{tabular}{lr}
\hline Substance & $\mathbb{K}_{\mathrm{f}}^{\circ} / \mathbf{k J} \cdot \mathrm{mol}^{-1}$ \\
\hline $\mathrm{Ag}^{+}(a q)$ & $77.1$ \\
$\mathrm{Cl}^{-}(a q)$ & $-131.2$ \\
$\mathrm{AgCl}(s)$ & $-109.8$ \\
$\mathrm{Br}^{-}(a q)$ & $-104.0$ \\
$\mathrm{AgBr}(s)$ & $-96.9$ \\
\hline
\end{tabular}