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 +xare made at the end of the month for the second 12 months.

Payments of$450 + 2xare made at the end of the month for the last 12 months.

What shouldxbe if the nominal monthly rate is 13.8%?

Thank you!