Final answer:
When the a-value of a parabola is negative, the parabola opens downwards and the vertex becomes the highest point on the graph.
Step-by-step explanation:
When the a-value of a parabola becomes negative (a < 0), it means that the parabola opens downwards. This is in contrast to when a is positive, and the parabola opens upwards. The vertex of the parabola remains the highest point on the graph in this condition and the shape is concave down. As the absolute value of a increases, the parabola becomes narrower, indicating a steeper incline on either side of the vertex.