Hi,
I am surprised no one answered this yet.
Solve equ 3-70 for I1(s), then insert that solution into equ 3-69, call that new equation A1 and it only has I2(s) in it now. Then solve equ 3-71 for I2(s), then insert that into the new equation A1, solving for Eo/Ei.
So the whole deal here is to solve for I1 and I2 simultaneously.