解
x/2=y/3
∴x=2y/3
∴(x²-2xy-3y²)/(x²-6xy-7y²)
=(x-3y)(x+y)/(x-7y)(x+y)
=(x-3y)/(x-7y)
=(2y/3-3y)/(2y/3-7y)
=(-7/3)×(-3/19)
=7/19