y=x^2-(a+2)x-9
=[x-(a+2)/2]^2-(a+2)^2/4-9
顶点在y轴上,则顶点横坐标等于0
则(a+2)/2=0
a=-2
y=x^2-9
y=0,x^2=9
x=3,x=-3
所以和x轴的交点坐标是(-3,0)和(3,0)