How to find a zero of this function?

[Clunk_Head]

I have many input parameters and a function in the end of the following code. The unknown parameter is the "Fi". I tried to find a zero of the last function, but I could make only an iteration for this problem. Can you please help me to find a better function for determining "Fi"? Thank you :-)

Can you quantify better, please?
Do you want something faster or do you need it to be simpler code, or do you want more pythonic code?
It's hard to tell.
Also, please describe your math.