To eliminate terms in expression simplification, you're looking for expressions that equal zero when added together with a given term.
In your example, the term \( 3x \) would be eliminated if you add \( -3x \) (or \( -1 \times 3x \)). Therefore, you could fill in the blank with:
- -3x
So, \( 3x + (-3x) = 0 \).
In general, for any term \( ax \), the term that would eliminate it would be \( -ax \).