For example, there is a known ellipse and a straight line K with unknown slope, and they have two intersections. You can set it up, combine the ellipse equation with the straight line equation, and get a quadratic equation about the straight line K, and then use the conditions given by the problem and David's theorem (for example, you can calculate the slope of the straight line by giving you the abscissa or ordinate of the midpoint of the two intersections). If it is a little harder, it may involve action.

This is the most basic idea …