What Characteristics does north africa and sub africa of both regions have in common

The vertex form of a quadratic function:
(h,k) - the coordinates of the vertex 

If a>0, then the parabola opens upwards, so the vertex is the minimum of the function.
If a<0, then the parabola opens downwards, so the vertex is the maximum of the function.

a=-1<0
the vertex is maximum

a=1>0
the vertex is minimum