The difference in the rule of reflection for the point reflected across y = x is, the rule would be (x, y) ---> (y, x). So the x and y values would be switched around. On the other hand, the rule of reflection for the point reflected across y = -x is, the rule would be, (x, y) ---> (-y, -x). So the values would switch places and would have the opposite sign (If the number was positive, it would be a negative, and if it was a negative number, it would turn into a positive.)
