设双曲线是y^2/a^2-x^2/b^2=1
把M（-3,2√7）,N（-6√2,-7）代入
28/a^2-9/b^2=1
49/a^2-72/b^2=1
28/a^2-9/b^2=1乘以8
224/a^2-72b^2=8
减去49/a^2-72/b^2=1
175/a^2=7
a^2=25,b^2=75
y^2/25-x^2/75=1