Reflect point (a,b)(a, b)(a,b) across the line y=xy = xy=x. The image is:
(−a,b)(-a, b)(−a,b)
(b,a)(b, a)(b,a)
(−b,−a)(-b, -a)(−b,−a)
(a,−b)(a, -b)(a,−b)