To simplify the expression \( 3x + (x + 2) \) using the associative property, you can regroup the terms. The associative property states that when adding numbers, the way the numbers are grouped does not change their sum.
Here, we can regroup the terms as follows:
\[ 3x + (x + 2) = (3x + x) + 2 \]
Now, we can combine like terms:
\[ 3x + x = 4x \]
Thus, the expression simplifies to:
\[ 4x + 2 \]
So, the simplified expression is:
\[ 4x + 2 \]