So the coordinate is reflected through the line y=x, if the coordinate is (x,y) which has a different values the reflected coordinate would be (x,x) and they are the same because it is stated that y=x, so that values are the same with x.
The a point is refrected through the line y = x, the new point is found by interchanging the x and y-cordinate of the original point. i.e. if the original point is (a, b) and is refrected through the line y = x, the new point will be (b, a).
