Which presidential action of George Washington helped establish the principle that federal law is the supreme law of the land?

leading federal troops to put down the Whiskey Rebellion

Explanation:

The concept of federal law as the 'supreme law' views that he federal law should always overruled the state law in case they're contradicting one another.

At that time, the federal government wanted to create a law that increase the tax for whiskey products. The states of Pennsylvania decided to oppose this with their own law because they believe it will hurt their economy. This lead up the the Whiskey Rebellion.

George Washington eventually sent the federal troops to put down the rebellion. It indicates that he believed the federal law should be enacted even  if it's opposing the state law.

c]extended voting rights to african americans