recall that the energy of a capacitor can be thought of as the energy of the electric field inside the capacitor. The energy of the electric field is usually described in terms of energy density u, the energy per unit volume.
A parallel-plate capacitor is a convenient device for obtaining the formula for the energy density of an electric field, since the electric field inside it is nearly uniform. The formula for energy density can then be written as u=UV [where U is the energy of the capacitor and V is the volume of the capacitor (not its voltage)]. A parallel-plate capacitor has area A and plate separation d, and it is charged so that the electric field inside is E. Use the formulas from the problem introduction to find the energy U of the capacitor.