x/y=-2
则
x=-2y
则
原式
=(x²-2xy-3y²)/(x²-6xy-7y²)
=(x+y)(x-3y)/(x+y)(x-7y)
=(x-3y)/(x-7y)
=(-2y-3y)/(-2y-7y)
=(-5y)/(-9y)
=5/9