The vertex form of a parabola is `y=a(x-h)^2+k` where the vertex is (h,k).
If you know the vertex you will need to know one more point to find a.
For example, suppose you are given a parabola with vertex at (3,4) going through the point (2,6). (Sometimes you are given the additional point; you might have to read it off of the graph. Or perhaps you will be told about an intercept.)
Then you know `y=a(x-3)^2+4` . To find a substitute the x and y values of the other known point. Here x=2 and y=6:
`6=a+4 ==> a=2` so the function was `y=2(x-3)^2+4` .