Equation / function solver setup

I am working on a sheet that uses strain compatibility to determine the neutral axis locations of a compression member with given axial forces.

I have attached the part of this sheet that I need help with. I believe I have everything set-up correctly for the constraints and have canceled units within the equations, but now I need to add a solving step. This should be able to work using a function definition with the Find function, but I have not been able to set this up correctly. I am also getting stuck trying to use multiple axial forces (in a vector) and getting an output of the corresponding neutral axis locations.

Any help would be greatly appreciated!

Thanks,

Karl