You can put this solution on YOUR website! The formula for a horizontal parabola with vertex at the origin is . 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 .
Directrix relies on finding 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 .
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".
(