count the gap between 24 and 28. then 28 and 33. and 33 and 39. it starts at going up by 4, then 5, then 6. following this pattern, you would count up 7 from 39 to get your next number
it's the same thing with 13, and 15. the gap between them is 2. that means the next one would be a gap of three, resulting in 18. that also means the one behind it would be a gap of 1, making the missing number 12.
