84
from 4 to 7 , they added 3
from 7 to 12 they added 5
from 12 to 19 they added 7
from 19 to 28 they added 9
3, 5, 7, 9.... They are simply adding odd numbers to each number. So after 9 would be 11, so you add 11 to 28 and get 39, and so on...
39 + 13= 52
52 + 15 = 67
67 + 17 = 84
84 is the ninth term, so that is your answer.
