An investment of $20,000 in new equipment will generate income of $7000 per year for 3 years, at which time the machine can be sold for an estimated $8000. If the companys MARR is 15% per year, should it buy the machine?

I know thati have to solve for the interest rate and the interest rate should be 18.2% peryear, but i dont know how to find the interest rate using linear interpolation.