I just showed. It's the same distance as from the vertex to the directrix.

. |p| is the distance from the vertex to the focus.

'a' is the coefficient of the x^2. Remember, we had -1/6 on the one I graphed. So,

It's 3/2 units from the focus to the vertex or from the vertex to the directrix.

If p<0, then the parabola opens down. If p>0, then it opens up.

Of course, they can be horizontal too. In that case, you have

instead of

If it's a horizontal parabola, then if p>0 it opens to the right and if p<0 it opens left.