A new employee charged $$9840$ on his credit card to relocate for his first job. After noticing that the interest rate for his balance was $27\%$ compounded monthly, he stopped charging on that account. He wishes to pay off his balance in $4$ years using automatic payments sent at the end of each month.

a$.$ What monthly payment must he make to pay off the account at the end of $4$years$?$

b$.$ How much total interest will he have$$ paid?