-x+4=k/x
x^2-4x+k=0
x1+x2=4
x1*x2=k
X1^2+X2^2+8*X1*X2-X1^2*X2^2=0
(x1+x2)^2+6x1*x2-x1^2*x2^2=0
16+6k-k^2=0
k^2-6k-16=0
k=8 or k=-2
-x+4=k/x
x^2-4x+k=0
x1+x2=4
x1*x2=k
X1^2+X2^2+8*X1*X2-X1^2*X2^2=0
(x1+x2)^2+6x1*x2-x1^2*x2^2=0
16+6k-k^2=0
k^2-6k-16=0
k=8 or k=-2