What are you referring to as "ultimate system"? UltimateTax is software for professional tax preparers. Are you a tax professional? This forum is for users of TurboTax. If you used other software, you have to look elsewhere for support for the software that you used.
If you used TurboTax, what do you mean by skipping a line?
Who is the letter from: IRS? State tax department? (If state, what state?)
What year is the letter about?
What reason does the letter give that you owe $750?
Did you purchase Audit Defense for the year in question?
