Finding a maximum height. (Polynomial function)

Hi, I have the correct answer but I am not sure how I got it:

The path of a diver is given by

y= -4/9x^2 + 24/9x + 12 where y is the height (in feet) and x is the horizontal distance from the end of the diving board (in feet). What is the maximum height of the diver?

Ok, so I used this formula : -b/2a (the vertex) to get x = 3. Apparently that is the answer, but I'm curious as to why they wouldn't plug 3 as the x value to get the height. Any help is appreciated. Thanks.