设（x1,0）,(x2,0),(0,y1),x2>x1
(x2-x1)*(-y1)=12*2
y = a（x-h）^2 + k a>0
h=-1,k=-8
0=a(x1+1)^2-8
0=a(x2+1)^2-8
y1=a-8
综上解得a=2
则y=2(x+1)^2-8