The law that applies to situations where state and federal laws disagree is called the supremacy clause, which is part of article VI of the Constitution. The supremacy clause contains what's known as the doctrine of pre-emption, which says that the federal government wins in the case of conflicting legislation. Basically, if a federal and state law contradict, then when you're in the state you can follow the state law, but the feds can decide to stop you. When there is a conflict between a state law and federal law, it is the federal law that prevails. For example, if a federal regulation prohibits the use of medical marijuana, but a state regulation allows it, the federal law prevails.
Okay.
But this is conflict between state and local law.