Electrostatic Potential and Capacitance Part 1

A parallel plate capacitor whose capacitance C is 14pF is charged by a battery to a potential difference V=12V between its plates. The charging battery is now disconnected and a porcelain plate with K=7 is inserted between the plates, then the plate would oscillate back and forth between the plates with a constant mechanical energy of ......... pJ.
(Assume no friction)