Answer
$\approx 4,468\text{ km}$
Work Step by Step
Find the measure of the central angle between $28^oN$ and $12^oS$.
The $12^oS$, since it goes clockwise from the equator, can be represented by $-12^o$,
Thus, the measure of the central angle between the two cities is:
$=28^o-(-12^o)
\\=28^o+12^o
\\=40^o$
Convert the angle measure to radians by multiplying it by $\dfrac{\pi}{180^o}$ to obtain:
$\require{cancel}
=40^o \times \dfrac{\pi}{180^o}
\\=\cancel{40^o}^2 \times \dfrac{\pi}{\cancel{180^o}9}
\\=\dfrac{2\pi}{9}$
The distance between the two cities, which $s$, is is given by the formula $s=r\theta$ where r = radius and $\theta$= central angle measure in radians.
Therefore, the distance between the two cities is:
$s=r\theta
\\s=6400 \text{ km} \times \dfrac{2\pi}{9}
\\s\approx 4,468\text{ km}$