The answer is 39,393.
This is because their are five spots:
_ _ _ _ _
If the first third and fifth are multiples of three here are some possible values
3,6,9
But since it is less than 40,000
the answer can only be 3, b/c 6 is greater than 4 and so is 9. The numbers cant be more than single digit multiples or else the number would be much bigger than 40,000.
SO FAR...
3 _ 3 _ 3
Now that you know those digits are 3, the second part says the remaining digits are 6 more than the one figured out. So six more than 3 is 9. Filling in the numerals for the final number, it comes out to be 39393.