Let:
[tex]\begin{gathered} A=(2,-1) \\ B=(-1,2) \\ C=(2,3) \\ D=(4,2) \end{gathered}[/tex]
After a reflection across the line y = x:
[tex]\begin{gathered} A->(y,x)->A^{\prime}=(-1,2) \\ B->(y,x)->B^{\prime}=(2,-1) \\ C->(y,x)->C^{\prime}=(3,2) \\ D->(y,x)->D^{\prime}=(2,4) \end{gathered}[/tex]
