In the Extended Projective Plane, let a be the line with equation x=0, b the line with equation y=x-4, and c with equation y=-x+4. Find the equation of the line d such that H(a,b;c,d).
I understand the process to determine this line d. I would intersect the lines with a line x such that x has equation y=0 and then use the cross ration formula to determine the point D and then find the line through D concurrent with a,b, and c. However, I am not understanding how this process would work with this problem as the three lines a,b, and c are not concurrent and I believed that this was required for four lines to form a harmonic quadruple of lines.
Any help is appreciated.