If the concentration of OH^ - ions in the reaction Fe(OH)_3(s)...

Key Idea : The concentration of solids taken to be unity. $$\begin{aligned} & \mathrm{Fe}(\mathrm{OH})_3(s) \rightleftharpoons \mathrm{Fe}^{3+}(a q)+3 \mathrm{OH}^{-}(a q) \\ & \therefore \quad K=\left[\mathrm{Fe}^{3+}\right]\left[\mathrm{OH}^{-}\right]^3 \end{aligned}$$ Hence, if $\mathrm{OH}^{-}$ion concentration is decreased by $\frac{1}{4}$ times, then equilibrium concentration of $\mathrm{Fe}^{3+}$ will increase by 64 times.