What is meant by energy density of a parallel plate capacitor?

Using the above expression, show that energy density of a parallel plate capacitor is [1/2(epslion_0 E^2)] where .E. is electric field between parallel plates.

A dielectric slab is placed between the plates of a parallel plate capacitor. Its capacitance.

How capacitance changes if the distance between the plates of a parallel plate capacitor is halved?