You need to review the letter to see why they are assessing you the additional tax. Do not ignore it, ti will not go away. Very frequently, these notifications are sent out based on partial information that the IRS has and the actual amount of tax liability is much less than they are billing you for. If you paid someone to prepare your return, take the letter to them. If they were not a CPA or Enrolled Agent they cannot represent you before the IRS so you may need to retain someone who can work with the IRS.