To simplify the expression \( 3x + (x + 2) \) using the Associative Property, we'll first rearrange the terms.
The Associative Property allows us to regroup the terms without changing the sum. Here, we can rewrite the expression by combining \( 3x \) and \( x \):
\[ 3x + (x + 2) = (3x + x) + 2 \]
Now we can simplify \( 3x + x \):
\[ 3x + x = 4x \]
Substituting this back into the expression gives us:
\[ (3x + x) + 2 = 4x + 2 \]
Thus, the simplified form of the expression \( 3x + (x + 2) \) is:
\[ \boxed{4x + 2} \]