If E is the electric field of a cell membrane, then find the electric field inside and outside of the cell.

Well, what did you find? PS- No homework questions answered unless you try to solve.

I know in a air-filled parallel capacitors, E = 0 outside.
In a cell, E is positive outside and negative inside. Also, the dielectric constant is 8.7.
I need at least a hint to start, please. I think I have to use Gauss' law.
Thanks.

You do realize that you posted this question into a "Residential Electrical DIY" forum, not a Physics or Electronic Engineering forum, correct.

I'm going to ask to have your question moved into a more appropriate forum.