Question: Find the length of the segment $AB$.
The length is [ ] . (Type an integer or a decimal.)
Graph: A horizontal number line is shown across the center; points are marked $C$ at $-5$, $D$ at $4$, $A$ at $5$, $E$ at $7$, and $B$ at $9$. The segment $AB$ is the distance from $A$ to $B$ on the center-right of the graph.
1. **State the problem:** We need to find the length of the segment $AB$ on a number line where $A$ is at $5$ and $B$ is at $9$.
2. **Formula used:** The length of a segment on a number line between points $x_1$ and $x_2$ is given by the absolute difference:
$$\text{Length} = |x_2 - x_1|$$
3. **Apply the formula:** Here, $x_1 = 5$ and $x_2 = 9$.
$$\text{Length} = |9 - 5|$$
4. **Calculate the difference:**
$$\text{Length} = |4| = 4$$
5. **Interpretation:** The length of segment $AB$ is $4$ units.
**Final answer:**
$$\boxed{4}$$