Equation of the Directrix

Question

Hi I am confused about what the equation of the directrix is. Can you help me out please?

Answer 1

Remember that the focus and directrix are equidistant from the vertex of the parabola. So you need to know where the vertex is and the distance from the vertex to the focus which will be the same as the distance from the vertex to the directrix. So if we have an upward opening parabola whose vertex is (2,-2)and the focal length is 3 then the focus is at (2,1) and the directrix is y=-5