What were the purpose of Darius and Xerxes in the Persian Empireand how did they govern a large area of it?
http://www.google.com/search?q=darius+persian+empire&ie=utf-8&oe=utf-8&aq=t&client=firefox-a&rlz=1R1GGGL_en___US357
http://www.google.com/search?hl=en&client=firefox-a&rlz=1R1GGGL_en___US357&hs=sXI&q=xerxes+persian+empire&aq=0c&oq=x+persian+empire&aqi=g-c1
You need to learn to go back into your textbook or study materials to find these answers – or learn to conduct simple searches, such as I did above. Enter each of the terms you listed until you find a definition that gives you the information you need.
http://www.answers.com
http://en.wikipedia.org/wiki/Main_Page
http://www.google.com