To simplify the expression \( 3x + (x + 2) \) using the Associative Property, we will first rearrange and group the terms.
The Associative Property states that the way in which numbers are grouped in addition does not change the sum. This means we can regroup the terms however we like.
Starting with the original expression:
\[ 3x + (x + 2) \]
We can remove the parentheses and regroup the terms:
\[ 3x + x + 2 \]
Now, we can combine the like terms \(3x\) and \(x\):
\[ (3x + x) + 2 = 4x + 2 \]
Thus, the simplest equivalent form of the expression \(3x + (x + 2)\) is:
\[ \boxed{4x + 2} \]