Write the standard form of the parabola with its vertex at (0,0) at te origin and directrix x= -3

Since the directrix is at x=-3, the parabola open toward the positive x-axis and has the equation



The distance from the vertex to the directrix is the same as the distance from the vertex to the focus. P is this distance in the above equation. We can easily see that p=3.

Plug in p=3:



That's it.

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