Problem 20.23

20.23.
We want to find the potential at a distance of 2R away from the ring and subtract it from the potential at the center.
$\Delta V = V_{2R} - V_{0}$
$\Delta V = \frac{kQ}{\sqrt{R^{2} + (2R)^{2}}} - \frac{kQ}{R}$
$\Delta V = \frac{kQ}{R} (\frac{1}{\sqrt{5}} -1)$
$\Delta V = -.553 \frac{kQ}{R}$