Coxeter- Exercise 2.5.2
From the construction for 2.5A, we know that F=AB·PQ. In that case the two vertices on line f should be P and Q. We also know that QR,QS,RS pass throught A,B,C. If Q is at the top, then R is on the left, and S is on the right. If P is at the top, then S is on the left,and R is on the right.