Question 825620
Being brief,  you could use the definition for Parabola and derive the equation, or refer to the formulation described in http://en.wikipedia.org/wiki/Parabola , under the sections of "Equations, Cartesian" or "Equations in Cartesian Coordinates".


Note that the directrix is on the other side of the vertex than the focus.  The vertex is between the focus and the directrix.