Vector calculus - this is incredibly useful for starting out in the subject of Electromagnetism. Many people find this subject particular difficult, and the way to make sure you ace this is by reviewing all the details from vector calculus by going through the first chapter of Graduate EM books (Franklin / Jackson / Zangwill perhaps). This will be particularly helpful for you because given the time constraints put on everyone during the quarter system, you do not get the chance to fully go through this