Question 709087
Assuming this parabola has a vertical axis, we can find the other coordinate of the minimum.  Zeros at x=0 and x=6 mean that the symmetry axis runs directly in the middle, at x=3.  Vertex (the minimum point in this case) is at (3,-9).



You now have something which in standard form can be written as
{{{y=a(x-3)^2-9}}}, which has only one undetermined value, "a".  The x and y are variables but would ordinarily remain as variables.  You can still use the given points (0,0) and (6,0) to help find the still unknown "a".  



Known points for this graph: (0,0), (6,0), (3,-9).



The rest of the description of this solution is still left unfinished, but maybe you can decide what to do the rest of the way.