Can you share what the actual / proper calculation would be (like with a spreadsheet)? It’d be easier to see the math laid out before figuring out the expression.
I’m not personally familiar with loan calculations, but I know there is a formula type:
“Calculate Formula” → “Loan Payment”. Here I pulled it up on a text element:
and then I chose “Loan Payment”
and from there choose which fields the calculation is pulling from: