If you remember, there is a divisibility rule of 9 that it would divide any number if the sum of digits of the number add up to one of its multiples. So here,

5+2+* = multiple of 9


Now, it may be any number that would add up to make a multiple of 9. So, 2 could be one number which would add up to 9 which is, of course, a multiple of 9. So, * can be replaced by 2 and the number thus becomes 522.
2 2 2
2 fits the blank
so 522 is the number