This type of essay answer is pretty short.
You play out the equation and say what each part of the equation means. The vertex in this case, because it's a downward facing parabola, is the highest point on the graph. Now since they're kicking a ball, the vertex will be the highest point that the ball will go up to.
Hopefully this helps some. For full points you'll probably have to add a bit of padding and further explanation.
