College Physics (4th Edition)

$U = 2.4\times 10^{-6}~J$
We can find the energy stored in the capacitor: $U = \frac{1}{2}~Q~V$ $U = \frac{1}{2}~(0.020\times 10^{-6}~C)(240~V)$ $U = 2.4\times 10^{-6}~J$