设直线l的方程为：y=x+m
代入双曲线C中得
x^2/a^2-(x+m)^2/b^2=1
即：(1/a^2-1/b^2)x^2-2m/b^2*x-1-m^2/b^2=0
x1+x2=(2m/b^2)/(1/a^2-1/b^2)=2ma^2/(b^2-a^2)=2
y1+y2=x1+m+x2+m=2+2m=6
即m=2
从而4a^2/(b^2-a^2)=2
3a^2=b^2
e^2=c^2/a^2=(a^2+b^2)/a^2=4
C的离心率：e=2