A parallel-plate capacitor is made from two plates 12.0 {rm cm} on each side and 4.50 {rm 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. (See the figure below .) An 18.0 {rm V} battery is connected across the plates. What is the capacitance of this combination?