(x^2-y^2)/(x+y)-(4x(x-y)+y^2)/(2x-y)
=(x-y)(x+y)/(x+y)-(4x^2-4xy+y^2)/(2x-y)
=(x-y)-(2x-y)^2/(2x-y)
=(x-y)-(2x-y)
=-x