If this straight line is a special position line, it is estimated that you will. If it is a general position line, then:

1. Take the midpoint (point C) of this straight line (named straight line A) first.

2. Make a horizontal line (nm) and a normal line (po) perpendicular to the straight line passing through the midpoint, and the plane where the straight line mn and the straight line po are located is a vertical plane.

3. Find the intersection of a vertical plane and a straight line (the problem becomes finding the intersection of a straight line and a plane, which should be in the book).

This is my own imagination, I don't know if it's right, I hope it will help you.