There is no 3645-C form or letter that I can find on the IRS website. Not even a reference to it. Is this a state thing maybe? However, there is an IRS Form 3645 which is used to figure the penalty for a business with employees that did not do proper reporting of the 940 on employee wages and the employer contributions on the social security and Medicare tax. My screen shows "unknown" for whatever version and flavor of the TurboTax program you're using. So without knowing what type of return the letter is for, I'm making wild guesses here based on no facts. Generally, the 3645 letter relates to a 1065 Partnership Return, or an 1120-S or 1120 Corporate return.
