To rewrite the expression \( 3x(x + 3) \) using the Distributive Property, you multiply \( 3x \) by each term inside the parentheses:
\[ 3x(x + 3) = 3x \cdot x + 3x \cdot 3 \]
This simplifies to:
\[ 3x^2 + 9x \]
So, the expression rewritten using the Distributive Property is \( 3x^2 + 9x \).