To rewrite the expression \( 3x(x + 3) \) using the distributive property, you need to distribute \( 3x \) to both terms inside the parentheses:
\[ 3x(x + 3) = 3x \cdot x + 3x \cdot 3 \]
Now, simplify the terms:
\[ 3x^2 + 9x \]
So, the expression \( 3x(x + 3) \) rewritten using the distributive property is:
\[ 3x^2 + 9x \]