Continuing with the Hint 4, you can now draw the images of all the midsegments of the triangle ACE. Each midsegment and the corresponding side of the triangle define together a pair of parallel lines the images of which meet at the focal plane.

Alternatively, if you have found the path of the Hints 3-4-5 difficult to follow, you may look into projective harmonic conjugates.

*Please submit the solution to this problem via e-mail to physcs.cup@gmail.com.** *