Problem:
Pat's twins, Sherry and Katie, finished their first year of school at an accredited university in 2013. She paid $9,000 in qualified educational expenses for Sherry and $3,000 of qualifying expenses for Katie. Pat is filing as a head of household with an AGI of $84,000.
Required:
Question: What amount of American opportunity credit may she claim?
Note: Explain all steps comprehensively.