Try to derive an estimate
You will have to find out the missing parameters by yourself.
For the first step, identify the linear distance between Koblenz and Windhoek with the diameter of the earth and suppose it to be perpendicular to the line earth - sun.
For the second step, determine the linear distance between the cities but keep it perpendicular to the line earth - sun.
For the, much more difficult, third step, try to determine the angle between the both lines Koblenz - Windhoek and earth - sun, respectively.