In this question , can I write the electric field on the right of the rightmost plate by distributing the charge on each surface and writing electric field due to each surface? I'm really confused between the two formulas (sigma)/2£ and (sigma)/£ ..... How to decide which formula to use? Please help.
Yes you can ,
But since you’ll distribute the charge , and hence you’ll be writing electric field due to each surface individually and hence use 1/2