and how can I apply it to a problem question
Basically EU law trumps national law.
In a case question you would usually have some national law and some EU law, both saying different things. According to Van Gend en Loos the EU law one takes priority so the national law should be ignored and effectively deleted. Just make sure the people in this case include a person/company and the state (vertical relationship), and not two individuals/companies (horizontal direct effect).