A free electron is placed between two large, parallel, non-conducting plates that are horizontal and 2.3 cm apart. One plate has a uniform positive charge; the other has an equal amount of uniform negative charge. The force on the electron due to the electric field E between the plates balances the gravitational force on the electron" What are

(a) The magnitude of the surface charge density on the plates and

(b) The direction (up or down) of E?