b
$\frac{1}{2} m V_{1}^{2}-\frac{K Q q}{r_{1}}=\frac{1}{2} m V_{2}^{2}-\frac{K Q q}{r_{2}}$
$\mathrm{mV}_{1} \mathrm{r}_{1}=\mathrm{L}_{0}$
$\mathrm{mV}_{2} \mathrm{r}_{2}=\mathrm{L}_{0}$
Solving eqns:
$L=\sqrt{\frac{m r_{1} r_{2} Q q}{2 \pi \varepsilon_{0}\left(r_{1}+r_{2}\right)}}$