To simplify the expression \( 18x + 12 - 15x \), you can combine like terms.
First, combine the terms involving \( x \):
\[ 18x - 15x = 3x \]
Now, rewrite the expression with the combined terms:
\[ 3x + 12 \]
So, the simplified form of the expression \( 18x + 12 - 15x \) is:
\[ \boxed{3x + 12} \]