
$\frac{\mathrm{k}(2 \mathrm{qa})}{\mathrm{x}^{2}}=\frac{\mathrm{k}(4 \mathrm{qa})}{(\mathrm{R}-\mathrm{x})^{2}}$
$R-x=\sqrt{2} x$
$x=\frac{R}{\sqrt{2}+1}$

Reason : For a system of positive test charge and point charge electric potential energy $=$ electric potential.
