The potential energy stored by a capacitor is given by: [tex]U= \frac{1}{2} \frac{Q^2}{C} [/tex] where Q is the charge stored in the capacitor and C is the capacitance.
In this problem, [tex]Q=30.0 \mu C=30.0 \cdot 10^{-6} C[/tex] [tex]C=5.0 \mu F=5.0 \cdot 10^{-6} F[/tex] Therefore, the potential energy stored in the capacitor is [tex]U= \frac{1}{2} \frac{(30.0 \cdot 10^{-6}C)^2}{5.0 \cdot 10^{-6}F}=9.0 \cdot 10^{-5} J [/tex]
