The image is too blurry for me to be sure of the axis labelling. But you can see it clearly, right? So please reply with the plot points you've gathered from the graph, including the vertex and the x-intercepts.

You know that the vertex form of the equation is y - k = a(x - h)^2. You've found the vertex. You've found at least two more points. You plugged in the vertex and one of the other points, and solved for "a". What equation did you get?