e=c/a=2
c=2a
c^2=a^2+b^2=4a^2
b^2=3a^2
焦点在X轴上,设为x^2/a^2-y^2/b^2=1
(2根号3,-3)代入得12/a^2-9/(3a^2)=1, a^2=9
方程x^2/9-y^2/27=1