I calculated this using flat interest
12 years x 12 months in a year = 144 payments
144 payments x $680= $97,920 total to be repaid
$97,920- $72,000 loan amount = $25,920 amount more than loan
($25,920/ $72,000) x 100 = 36%
Divide amount over loan by loan amount then multiply by 100 to get the total interest rate
36% / 12 years = 3% per year
Divide the total interest rate by the number of years to get the yearly interest rate