Parabola has following properties.
1.The distance from the point on the parabola to focus is equal to the distance to directrix. 2.We define the coordinates of F,P and H as (a, b), (x, y) and (x, b) respectively. Since PF = PH (x  a)^{2}+(y  b)^{2} = (y + b)^{2} The equation of parabola is y = (x  a)^{2}/4b. 
