In projective geometry, there's no concept of "between". So I think there's no clear concept of what "a point S inside" means. But in this question, we can think it as in Euclidean plane. In that case, draw the triangle PQR first, then randomly choose a point inside PQR. PQRS is our quadrangle, and the diagonal points being A,B,G. Connect A,B, we get the line g. C=RS·AB, F=PQ·AB.