Intercept form is the following:
where p and q are the x coordinates at which the parabola crosses the x-axis (that is, the x-intercepts).
Our job is to find the values of a, p and q.
In this case, p=9 and q=1 since you are told in the problem these are the x-intercepts.
Plugging in the values for q and p we get:
Now we just need to find the value of a
In the problem we are told that this parabola passes through x=0 and y=-18. So we plug in those values into our expression and solve for a:
Finally, we plug in the value of a into our expression:
Hope this helps.