17.4% APR is 17.4/12% per month=1.45% or 0.0145.
After 1 month 843.12 will grow to 855.35
Take off 330 to leave 525.35.
After another month this will grow to 532.96.
Take off 330 to leave 202.96.
After another month this will grow to 205.91.
So the total cost will be 2×330+205.91=865.91.
