Answer
$\approx 11,952\text{ km}$
Work Step by Step
Find the measure of the central angle between $72^oE$ and $35^oW$.
$35^oW$ can be represented by $-35^o$ as it is measured going to the left.,
Thus, the measure of the central angle between the two cities is:
$=72^o-(-35^o)
\\=72^o+35^o
\\=107^o$
Convert the angle measure to radians by multiplying it by $\dfrac{\pi}{180^o}$ to obtain:
$\require{cancel}
=107^o \times \dfrac{\pi}{180^o}
\\=\dfrac{107\pi}{180}$
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:
$s=r\theta
\\s=6400 \text{ km} \times \dfrac{107\pi}{180}
\\s\approx 11,952\text{ km}$