Geometric series problem .

A house buyer borrows RM 50000 from a bank to buy a house which costs RM 70000 .The rate of interest charged by the bank is 9% per annum ,and is calculated based on the amount outstanding at the begining of each year . The house buyer is required to repay his loan in monthly installments for a period of 15 years . Assuming that the rate of interest is fixed for the entire duration of the loan , find the amount per month he has to repay the bank ?

My attempt :

It forms a sequence as follows :

The 15th term ,

which is the amount he has to pay by the end of 15 years .

Thus , every month he has to pay

My answer is obviously wrong . I wonder where my mistake is .. Thanks to the one who help me out ..