Answer
$U=3.89\times 10^{-21}J$
Work Step by Step
$q=\frac{Q}{12}=\frac{2.16\times 10^{-13}}{12}=1.8\times 10^{-14}C$
Now;
$U=12\times(\frac{Keq}{R})$
We plug in the known values to obtain:
$U=12\times (\frac{9\times 10^9\times 1.6\times 10^{-19}\times 1.8\times 10^{-14}}{0.08})=3.89\times 10^{-21}J$
