Implement Mathematical equations

I need the Excel equation in the this link to programmed in Kotlin or java
The Excel code is in the answer of " Stefano "
https://stackoverflow.com/questions/34867134/excel-vba-compute-an-approximate-interest-rate-of-a-french-amortization-schedul/68302682#68302682

this will be s small job and you write a fixed number of hour to do this job