Like Isellhondas said, your $5000 of negative equity will get rolled into the new loan. most leases won't allow negative equity so financing might be your option.
So now you're going to finance a $35000 car but have a loan for $40000. The monthly payments will depend what interest rate you'll get, and how long of a term you'll choose. It all depends on your credit too.
If you finance $40000 over 60 months with $0 down at 9.9% your payment will be about $847/month.