Find, in cartesian form, an equation of the parabola whose focus and directrix are respectively (3, 0), x+5 = 0.

How can I find?

As I know y^2 = 4ax is a cartesian equation of the parabola. And every point on the parabola is equidistant from the focus (a, 0) and the directrix x+a = 0.

But here, there are 2 different values of a.