I don't see where your problem is - probably because you did not provide the Excel file. So I haven't scrolled further down but maybe you are trying to solve for two varaibles (a, a') and so you should use a solve block and not the root function as shown below.
In your sheet you still use your simple variable increment method mixed with root so I don't know what you try to achieve. Maybe, if you have further questions, you streamline yout sheet so it clearly shows whats unclear to you.
I also noticed that you used READEXCEL inside an Excel component - don't think that this makes much sense and you should decide either for the one or the other.
