If the answer doesn't have to be a whole number then it could be 18.857.
This is based on 1/7ths of each of the top line x the middle number 3.
i.e. 3/7ths of 56 = 24,
3/7ths of 21 = 9,
3/7ths of 44 = 18.857.
Somehow I think I may be wrong but at least the logic is there.