(x 1+x2)(x 1-x2)/a^2=-(y 1+y2)(y 1-y2)/b^2
Therefore: (y1-y2)/(x1-x2) =-(b 2/a 2) * (x1+x2)/(y1+y2).
Where: x 1+x2 = 2x0, y 1+y2 = 2y0.