With vacuum between its plates, a parallel$-$plate capacitor has capacitance $4\mathrm{.}50$ F$.$ You attach a power supply to the capacitor, charging it to $3\mathrm{.}20$ kV$,$ and then disconnect it$.$ You then insert a dielectric sheet that fills the space between the plates. The potential difference between the plates decreases to $1\mathrm{.}10$ kV$,$ and the charge on each plate remains constant. Find the energy stored in the capacitor before you insert the sheet.