The images of Pa,b on the line y=−x is Q and the image of Q on the line y=x is R. Then the midpoint of PR is
a+b,b+a
a+b/2,b+a/2
a−b,b−a
0,0
The point Q is −b,−a and the point R is b,a. Therefore, the midpoint of PR is 0,0.