抛物线y=-1/3(x-h)²+K的顶点（h,k）;
y=-1/3(x-h)²+K=-1/3(x-h)²+h^2;
y=0; h^2=1/3(x-h)²; x-h=±√3h,
抛物线在x轴上截得的线段长是4√3,h=2;k=4