The united states has a federal system of government. what does that mean?

Ineed answers 5 and 6 on my social studies homework reply to this question i posted

What person was influential and trying to ease race relations in america during the early part of the twentieth century

Following archduke franz ferdinand's assassination, austro-hungarian leaders: a. aided the ottoman empire in its campaign to keep serbia under its control. b. executed nearly all prominent serbs living in austria-hungary c. threatened to invade serbia if serb leaders did not agree to a set of strict demands. d.. formed an alliance with great britain to protect austria-hungary from future attacks.