That's a sideways one; let's go back to first principals.
A parabola is the locus of points equidistant from a point called the focus and a line called the directrix.
It's almost always better to work with squared distance:
The squared distance from (x,y) to (3,0) is [tex](x-3)^2 + y^2[/tex]
The squared distance from (x,y) to x=-3 is [tex](x- - 3)^2[/tex]
Equating,
[tex](x-3)^2 + y^2 = (x+3)^2[/tex]
[tex]x^2 - 6x + 9 + y^2 = x^2 + 6x + 9[/tex]
[tex]x^2 - 6x + 9 + y^2 = x^2 + 6x + 9[/tex]
[tex]y^2 = 12x[/tex]
[tex]x = \frac{1}{12} y^2[/tex]
