When we reflect a point across y=x, the x and y coordinates switch out , that is (x,y) becomes (y,x) . SO for point (1,2), on reflection across y=x, (1,2) becomes (2,1) . And when we do reflection across y=-x, (x,y) becomes (-y,-x). Therefore point (2,1) on reflection across y=-x becomes(-1,-2) .
So the final answer is (-1,-2) .