Fixed Income CFA Level 1 Question - Macaulay's Duration

FV $1000
Coupon 7.17% semi annual
YTM 7.85%
Maturity 9.35833 years
Calculate Macaulay’s Duration.
Answer: 6.84

I am confused as to whether:
For Vo should I input I/Y as 7.85÷2=3.925, N=9.35833*2=18.71666, PMT=7.17÷2=3.585?

For V- : I/Y as 6.85÷2=3.425?
For V+ : I/Y as 8.85÷2=4.425?

then calculate modified duration and multiply by (1+ ytm/2)

my answer doesn’t match.