3) Whether the Supreme Court can declare laws of Congress to be unconstitutional
Marbury v. Madison was a crucial case in determining the ability for the Supreme Court to declare a law passed by Congress unconstitutional. Some of these laws have included Roe v. Wade (1973) (prohibiting abortion is unconstitutional) and Brown v. Board of Education (1954) (racial segregation in public school is unconstitutional). This process is known formally as judicial review.
I hope this helps! :)