If I understood the question correctly, you pay off a loan in six installments, each installement includes a repayment of principal and interest.
Interest is usually calculated on the outstanding amount, therefore, you should prepare a schedule which shows:
balance at the beg of period
repayment in the period (principal and interest)
interest accrued in the period (on the amount still outstanding)
balance at the end of period
Then you try and see different rates for the interest which one would fit your figures. Also, it is important to understand whether repayment happens at the beginnig or end of each period.