What specific items must be included in "support" when determining if individual qualifies for refundable portion of American opportunity credit?

Trying to make sure I do not over-estimate the cost of support when determining if my daughter's earned income was more or less than half of her support when determining if she is eligible for the refundable portion of the American opportunity credit.  I am helping her with her taxes and she had a couple of significant items that could cause her earned income to be less than half of her support.  She bought a car in 2018 - does she have to include the cost of the car in her calculation of "support"? She took education loans for her last semester of college.  Does she have to include the value of these loans in her calculation of "support"?  Is earned income based on gross income or income after taxes?