A loan of $18,000 is paid off in 36 payments at the end of each month in the following way:
Payments of $450 are made at the end of the month for the first 12 months.
Payments of $450 + x are made at the end of the month for the second 12 months.
Payments of $450 + 2x are made at the end of the month for the last 12 months.
What should x be if the nominal monthly rate is 13.8%?