a=-2,b=-5,c=7
x=-b/2a=5/2×（-2）=-5/4
y=(4ac-b²)/4a
=(-56-25)/(4×（-2))
=-81/(-8)
=81/8
即
对称轴为x=-5/4
顶点坐标为（-5/4,81/8)