Step-by-step explanation and answer:
A reflection across the line y=x (a line through the origin and at +45 degrees to the x-axis) is essentially an interchange of the x- and y-values, such as finding inverses.
For this reason, the reflection is usually represented by the operator
S_[x=y} : (x,y) ⇒ (y,x)
where _[...] represents subscript (smaller letters near the bottom.
