设A(x1,0)B(x2,0)C(0,c)
S=(x1+x2)/2c=6
x1+x2=-b/a=4,-b=4a
c=1/3
c-b^2/4a=-a
a+b+c=0,a=1/9,b=-4/9
所以y=x^2/9-4x/9+1/3