I agree with your Fy and Fx. Fx is the horizontal electric force on the 1.5μC charge.
The equation for the force on a charge, q, in an electric field, E is F = q x E.
You should be able to find the Electric field strength E.
Field strength is measured in Volts per metre so if you know the separation of the plates you should be able to find the voltage across them.
BUT... part (d) in your question states that the plates are 1 x 10^4 m apart. This is only 1/10 of a mm.... are you sure about this?
