The vertex occurs a the point (4,-9) The easiest way to do this is to draw the graph, or create a table of test values. When you do this, you will see as you increase the value of X, the value of Y continue to shrink until u hit X=4. The graph starts to increase in. So you know that is the vertex. In general, all functions of the form ax^2 +bx + c will have only one vertex. So you can apply this method.
