Write Coulomb`s law when two charges are placed in two different mediums

you can solve this question by calculating the equivalent permittivity between the two charges. Consider the charges and separated by two mediums in series having permittivities epsilon 1 and epsilon 2. The effective permittivity, from my knowledge, would be 2 times epsilon1 * epsilon2 divided by (epsilon1 + epsilon2). Just put this in the Force equation for 2 electric charges with epsilon substituted as the effective epsilon, you`ll get the answer.

You can assume a charge placed at the division of the dielectric spaces and then do it for each set. Then u will get the force on the charge in the middle then dividing it by the charge placed in the middle u will get the electric field . You can use this method to solve problems in a much simpler way