When reflecting over the line y = x, we swap the locations of the x and y coordinates. For example, the point (5,7) becomes (7,5)
This means we have the following reflections
A(9,-5) moves to A ' (-5, 9)
B(6,3) moves to B ' (3, 6)
C(-1,-6) moves to C ' (-6, -1)
This operation is commonly done when determining the inverse of a graph. This is because the inverse basically undoes everything the original function applies. So this "backwards" nature of things means that x becomes y and vice versa.