Write the parabola in standard form.
If the parabola is ((x-h)^2 = 4p(y-k)), the directrix is (y = k – p).
If the parabola is ((y-k)^2 = 4p(x-h)), the directrix is (x = h – p).
Identify the vertex ((h,k)).
Identify the value of (p).
Use the correct directrix formula based on the parabola’s orientation.
For a vertical parabola opening up or down, the directrix is a horizontal line.
For a horizontal parabola opening left or right, the directrix is a vertical line.
If the equation is in general form, rewrite it into standard form first.