#### Answer

$\color{blue}{s\approx 3500 \text{ km}}$

#### Work Step by Step

The central angle between the two cities is:
$=40^o-9^o=31^o$
Convert the angle to radians by multiplying $\dfrac{\pi}{180^o}$ to the angle measure to obtain:
$=31 \cdot \dfrac{\pi}{180^o}=\dfrac{31\pi}{180}$
RECALL:
The length of the arc $(s)$ intercepted by the central angle $\theta$ in a circle of radius $r$ is given by the formula:
$s = r\theta$, where $\theta$ is in radian measure.
Use the formula above to obtain:
$s=r\theta
\\s=6400 \cdot \frac{31\pi}{180}
\\s=3462.733236
\\\color{blue}{s\approx 3500 \text{ km}}$