Use a Scenario to Open Up Quadratic Eq in Vertex

Please see attachment. The graph is depicted from google source. I made up this scenario to use this as opening activity for Quadratic Equation of Vertex Form * * #5 key : y= -0.5(x- +4)^2+ 2 where a = -.5. For the value of a, I would ask students to find out any necessary hint in the graph to help them to solve for a. If they can use existed elements like (2, 0), (6,0) or (0, -6) to solve for value of a without me mentioning, they get extra credit in the assigment