The electric field on two sides of a charged plate is shown in the   figure. The charge density on the plate is given by

From the figure it is clear that the plate is placed in an external electric field. Let the electric field due to plate is E and Eo be the external electric field.Eo + E = 12 V/m            Eo – E = 8 V/mSolving equations,                                      E = 2 V/m  and                 Eo =     10 V/mNow electric field due to plate =  = 2= 4eo