If the vertex is the highest point on the graph what is it called

It's literally called the maximum or max.

If the parabola opens up, the vertex represents the lowest point on the graph, or the minimum value of the quadratic function. If the parabola opens down, the vertex represents the highest point on the graph, or the maximum value. In either case, the vertex is a turning point on the graph.

probability for j is 1/4

while

a is 1/6