To factor the expression \( 9z + 18 \) using the greatest common factor (GCF), we first need to identify the GCF of the two terms.
The GCF of \( 9z \) and \( 18 \) is \( 9 \).
Now we can factor out the GCF from the expression:
\[ 9z + 18 = 9(z + 2) \]
Thus, the factored form of \( 9z + 18 \) is:
\[ 9(z + 2) \]