Well, the form of a vertically opening (either up or down) parabola is

where (p, q) is the vertex. (We usually use (h, k) as the vertex, but you already have an "h" in the problem.)

It opens down, so a is negative. So let's write this as

and let's make "b" positive, just to remind us.

It has a vertex so your parabola is

Now, the parabola passes through (0, 0) so...

and it passes through (n, 0), so....

So we need to solve the simultaneous equations for h and b in terms of n:

So

Thus the parabola is:

-Dan