When the coefficient a is POSITIVE, the corresponding y values will be greater than the y expressed at the vertex. This means the graph will open upwards:
When the coefficient a is NEGATIVE, the corresponding y values will be less than the y expressed at the vertex. This means that the graph will open downwards:
Examine the graph for a = 2 and a = -2. The line y = 2 is where the two graphs intersect.
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