The Premium Tax Credit is a subsidy for low-income people who purchase health insurance from the Marketplace or exchange ("Obamacare"). Line C on the Smart Worksheet is "Household income for premium tax credit." It's the income amount that would be used to calculate the credit. But you probably have Medicare and didn't buy insurance from the exchange, so none of this applies to you. Line A of the Smart Worksheet, "Insurance thru exchange" says No. The amount on line C is just there in case it's needed. You can ignore it. It does not affect the calculation of your tax.
