$C=\kappa {{\varepsilon }_{{}^\circ }}\frac{A}{d}\Rightarrow \frac{{{C}_{2}}}{{{C}_{1}}}=\frac{{{\kappa }_{2}}}{{{\kappa }_{1}}}\times \frac{{{A}_{2}}}{{{A}_{1}}}\times \frac{{{d}_{1}}}{{{d}_{2}}}\Rightarrow 3=5\times \frac{d}{d+x}\Rightarrow 5d=3d+3x\Rightarrow x=\frac{2}{3}d$