Help pls

Well, how about sharing your solution with other people here. 

The r-coordinate is the distance between the point and the Earth's axis. In addition, the z coordinate is the distance between the point and the plane that contains the equator. Hence, traveling with constant latitude makes the r and z coordinate constant. Moreover, traveling with constant longitude makes the theta coordinate constant. 

ok thank you  :)

I will have to think about it a bit.