I just received a letter from irs for tax year2017. I forgot to report a 401k withdrawal. Should send in an amended return or just send the irs a check?

If you agree with the adjustment, just pay the bill. You don’t need to send an amended return, the IRS has already adjusted your file.

 However, you might want to prepare an amended return and double check the IRS figures. If you come up with a different amount of tax owed, you can mail the amended return and a check for the amount that you think is correct along with a letter of explanation, to the IRS office that sent you the original letter.   If you come up with the same amount, keep the amended return for your records but only mail the check.