a:b=1:2
所以b=2a
同理c=-3a
y=ax^2+2ax-3a
=a(x^2+2x)-3a
=a(x^2+2x+1-1)-3a
=a(x^2+2x+1)-a-3a
=a(x+1)^2-4a
顶点纵坐标=-4a=-8
a=2
y=2x^2+4a-6