To simplify the expression \( 3x + (x + 2) \) using the Associative Property, we can regroup the terms.
The Associative Property states that how we group numbers when adding does not change the sum. Thus, we can rewrite the expression as follows:
\[ 3x + (x + 2) = (3x + x) + 2 \]
Next, we simplify \( 3x + x \):
\[ 3x + x = 4x \]
Now we can substitute back into the equation:
\[ (3x + x) + 2 = 4x + 2 \]
Therefore, the simplest equivalent form of the expression \( 3x + (x + 2) \) is:
\[ \boxed{4x + 2} \]