To use the associative property to group like terms together in the expression \( 9x + (2x + 3) \), we can rearrange the terms without changing the order. This is how it can be done:
\[ (9x + 2x) + 3 \]
In this expression, the terms \( 9x \) and \( 2x \) are grouped together, while the constant \( 3 \) is kept separate.