there are two (x1, y1) B (x2, y2) they midpoint coordinates of p ((x1 + x2) / 2, (y1 and y2) / 2)

Also: arbitrarily point (x, y) with respect to (a, b) symmetric point (2a-x, 2b-y) then (2a-x, 2b-y) is also on this Function. We have f(2a-x)= 2b-y transposition, we have y= 2b-f (2a-x)