Question

What is the equation of the directrix of the parabola given by the equation (y − 3)2 = 8(x − 5)?

Answer 1

The vertex of the parabola is at (5,3) and its focal length is 8/4 = 2.
Since it opens to the right, the directrix will be a vertical line 2 units to the left of the vertex, so its equation will be x=(5-2), or x=3