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]
