x^2-4x+y^2+6y+13=0
x^2-4x+4+y^2+6y+9=0
(x-2)^2+(y+3)^2=0
x-2=0 x=2
y+3=0 y=-3
(x^2-2xy+y^2)÷(x^2-y^2)
=(x-y)^2/[(x+y)(x-y)]
=(x-y)/(x+y)
=(2+3)/(2-3)
=-5