The solution 349 is given, and its number 3, 4 and 9 determine how many times 13 can be subtracted:
The first subtraction is 3x13 (x100) = 3900
The second subtraction is 4x13 (x10) = 520
The third subtraction is 6x13 = 78
So the answer is 520, which is also what you get when you subtract 78 from 598 of course.