Two parallel plates have equal and opposite charges. When the space between the plates is evacuated, the

Question:

Two parallel plates have equal and opposite charges. When the space between the plates is evacuated, the electric field is E = 3.20 X 105 v/m. When the space is filled with dielectric, the electric field is E = 2.50 X105 V/m.
(a) What is the charge density on each surface of the dielectric?
(b) What is the dielectric constant?