What do you understand by the displacement-time graph? Draw a displacement-time graph for a girl going to school with uniform velocity. How can we calculate the uniform velocity from it?

(N/A) In the displacement-time graph,the time is taken on the $x$-axis and the displacement of the body is taken on the $y$-axis.
Since $\text{Velocity} = \frac{\text{Displacement}}{\text{Time}}$,the slope of the displacement-time graph gives the velocity.
For a girl going to school on a straight path in a given direction with a uniform speed,the displacement-time graph is a straight line passing through the origin (as shown in the figure).
The velocity of the girl can be obtained by finding the slope of the straight line $OP$.
Using the points $A$ and $B$ on the graph:
$\text{Velocity} (v) = \frac{\text{Change in displacement}}{\text{Change in time}} = \frac{BC}{AC} = \frac{40 \text{ m} - 20 \text{ m}}{4 \text{ s} - 2 \text{ s}} = \frac{20 \text{ m}}{2 \text{ s}} = 10 \text{ m s}^{-1}$.