Final answer:
A law the Federal government could strike down using the supremacy clause is one that contradicts federal law, such as state laws legalizing marijuana, which are at odds with the federal Controlled Substances Act.
Step-by-step explanation:
An example of a law that the Federal government could strike down with the supremacy clause is a state law that allows activities considered illegal under federal law. A prime instance is the legalization of marijuana in various states, despite it being illegal under the Controlled Substances Act, a federal law. The supremacy clause, as stated in Article VI of the Constitution, provides that federal laws have precedence over conflicting state laws when the federal law falls within the scope of national government's constitutional powers. A historical example is provided by the landmark Supreme Court case McCulloch v. Maryland (1819), which underscored the principle of national supremacy when it deemed that Maryland could not tax the Bank of the United States, as such action would violate the supremacy clause.