This question is difficult to answer without knowing what [[x]] means. The closest thing I can think of is this might be the nearest integer function, which is sometimes written as [x], ||x||, or nint(x). It is a function that rounds to the nearest integer (e.g. 4.7 becomes 5).

You may want to ask your teacher or consult your textbook to clarify what this notation means.

Assuming it's the nearest integer function, we notice there are no restrictions on what values x can take, so the domain is all real numbers.

We notice any value will be round a multiple of -3, so the range is

`{..., -6, -3, 0, 3, 6, ...}`