Line 6 is only to include the year-end balance of your own traditional IRAs. Do not include the balance in the inherited IRA.
Since the inherited IRA has no basis in nondeductible traditional IRA contributions, the entire gross distribution from the inherited IRA is taxable income.
If you made no distributions or Roth conversions from your own traditional IRAs, you'll only have a Form 8606 if you made a new nondeductible contribution for 2018 and there will no taxable amount calculated on Part I or Part II.
