Question 1100501
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The vertex is (3,-6) and the y value at the y 
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The what y value? at the y what?



All that can be done with what you have is {{{y=a(x-3)^2-6}}}, and "a" is unknown and seems to be only unknown with what you have so far described.  All you can do from this is:
{{{y=a(x^2-6x+9)-6}}}
{{{y=ax^2-6ax+9a-6}}}-------polynomial equation in descending degree of x.