What is the average unit price of Rock tracks?
SELECT AVG ( T2.UnitPrice )  FROM GENRE AS T1 JOIN TRACK AS T2 ON T1.GenreId   =   T2.GenreId WHERE T1.Name   =   "Rock"
average unit price refers to AVG(UnitPrice); Rock tracks refers to tracks with Genre Name = 'Rock'; JOIN is used to connect tables based on GenreId to match genres with tracks.