
Step-by-step explanation:
The general vertex form of the equation is
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where (h, k) are the coordinates of the vertex and a is a constant. In your case, the vertex is located at (0, -3) so your equation becomes

To find a, substitute the point (3, 0) into the equation
0 = a(3)^2 - 3 ---> a = 1/3
Therefore, the equation has the form
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