A parallel-plate capacitor is made from two plates 12.5 cm on each side and 4.20 mm apart. Half of the space between these plates contains only air, but the other half is filled with Plexiglas of dielectric constant 3.40. An 19.5 V battery is connected across the plates. If we remove the Plexiglas, but change nothing else, how much energy will be stored in the capacitor?