The amount of the first monthly payment of $6,351 that will go toward the principal is $5,560.17.
The amount of principal repayment is computed as follows:
The mortgage amount = $73,000
Annual interest rate = 13%
Monthly payment = $6,351 ($73,000 x 8.7%)
First monthly interest = $790.83 ($73,000 x 13% x 1/12)
First principal repayment = $5,560.17 ($6,351 - $790.83)
Thus, we can conclude that the first principal repayment is $5,560.17.