To simplify the expression \( \frac{9}{11z} - \frac{4}{11z} \), we can combine the two fractions since they have the same denominator.
\[ \frac{9}{11z} - \frac{4}{11z} = \frac{9 - 4}{11z} = \frac{5}{11z} \]
So, the simplified form is
\[ \frac{5}{11z} \]