The median number of students in each music class is 9 people. ,17,20,21 what should be the number in the blank?

If the list is of numbers of students in each music class, and if their median is supposed to be 19, then the blank must represent the number 18. The number must be lower than 20, and must be 19 when averaged with 20. Its value must be ...
  2*19 - 20 = 18


Altogether there are 6 numbers, so the median (19) will be the average of the middle two. Three of the numbers are more than 19, so the other three must be less than 19. The lowest of the upper 3 numbers is 20, so 19 must be the average of 20 and the missing number, which must be 18.