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.