If you are stuck, try starting with a smaller query. Can you figure out how to write a query to get the highest salary of an individual employee? How about the average salary for a single department? All the departments? etc
When you get stuck, post the last query you were able to write (that solves a smaller problem) and let us know what you got stuck on. This will let people give you more concrete help and will help you learn how to write queries. Handing out an answer doesn't do that.