There is some confusion here.
First, you wrote "how to solve annuity"
An annuity represents income per year (annum), not a payment schedule to buy a sailboat.
Second, the information on the web page indicates that an amortization schedule is requested, which makes sense since the problem solution is a payment schedule to buy a sailboat.
Third, the problem states "7.2% compounded monthly"; perhaps what was meant was calculated monthly.
Using the information in the problem, we can calculate an amortization schedule as follows:
A = Principal [ (r(1 + r)n / (1 + r)n - 1 ]
In this case, the Principal is Price - down payment = $31,821 - 0.15(31,821) ≅ $27,047
Then, using the above formula, the monthly payment will be $341.00 per month for 9 years (108 months).
$341.00/month x 108 months = $36,828
The total interest will be ≅ $36,828 - 27,047 ≅ $9,781
