parabolas

I have a worksheet that asks, "Write the equation in standard form for each parabola". Below are the questions with the given information. Please explain how to get answers?

1) Vertex (0,0), directrix y=8

2) Vertex (0,0), focus (5,0)

3) Vertex (0,0), focus (10, 0)

4) Vertex (0,0), directrix y= -4

5) Focus (-1,0), directrix x= 3

6) Vertex (4, 6), focus (4, -2)