The third proportional to x2-y2 and x-y is :

Let the third proportional to  $x2-y2$ and  $x-y$ be z. Then, 

$x2-y2:x-y::x-y:z$  

$=>x2-y2*z=x-y2$  

=> z= $\frac{x-y}{2}=\frac{x-y}{x+y}$