I believe the answer is 9.
the first line adds up to 5
the second line adds up to 14
the third line is unknown
the fourth line adds up to 26
the difference between line 1 and 2 is 9
if the third line adds up to 21 then the difference between the second and third will be 7 and the difference between third and fourth will be 5.
to get 21 on the third line we need to add a 9
the next line would add up to 29 for example because the difference between lines is decreasing by 2 each time.
I basically threw in a total from 15 upwards for line 3 until something made sense as a pattern.
I could be wrong.