How to you reflect a point over the line y=x

Draw the line y=x.  It has a slope of 1 and a y-intercept of 0.

Plot the given point (assuming that it's NOT on this line).

Draw a line perpendicular to y=x that passes thru the given point.  This perpendicular line will be y = -x.

How far is the original point from the line y=x?  The reflection of this point across the line y=x is the same distance from the line y=x, but on the opposite side of the line y = x.