Question 980831
The formula for a horizontal parabola with vertex at the origin is {{{4px=y^2}}}.  You can derive the formula using the definition of a parabola with the Distance Formula or you can find the derivation in a textbook.
|p|  is how far is the vertex from either the directrix or the focus.


Your example is  {{{9x=-5y^2}}}
{{{highlight_green(-(9/5)x=y^2)}}}.


Directrix relies on finding p.
{{{-9/5=4p}}}
{{{-9/20=p}}}


The directrix will be on the convex side of the parabola, and because yours is convex to the right, vertex at (0,0),  the directrix is  {{{x=9/20}}}.


The location of the focus is on the conCAVE side of the parabola, so  focus is at (-9/20, 0).  This may be helpful (?) in finding the "focal diameter".