what is the formula used by lending institutions?

when you borrow money there is a formula to determine the amount of monthly payments for a period of time.is it the rule of 72?whatever it is what is it expressed as a formula.
or is it PRT=x?

How do I find a ligitimate debt consolidation loan?

I am looking to take out a loan to pay off about 00.00 in credit card debt. Unfortunately, my credit score is pretty low. How do I find a reliable place to borrow money from? I understand that my interest rate will be higher. I'm just not sure where to begin looking or where to start for that matter. thanks.

Has anyone ever borrowed money from a personal lending web-site?

I read an article in the paper about these personal lending web-site like "the lending club" where you can borrow or lend money to people instead of a bank.
Have you ever tried it? If so, what's your opinion on it?