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.