To rewrite the expression \(3x(x + 3)\) using the Distributive Property, you will distribute \(3x\) to both terms inside the parentheses:
\[ 3x(x + 3) = 3x \cdot x + 3x \cdot 3 \]
Now, simplify each term:
\[ = 3x^2 + 9x \]
So, the expression rewritten using the Distributive Property is \(3x^2 + 9x\).