The significance of A(6,3) as a point in the x-y plane is a bit unclear because it doesn't sit on the line y=-x+3, which contains the point (6,-3). The only significance I can think of is the equation of the perpendicular from A on to the line. The equation of the perpendicular is y=x+b, where b=6-3=3 (substituting x=6 and y=3). y=x+3 and y=-x+3 meet at the point given by x+3=-x+3, i.e., x=0, so y=3. The solution would be (0,3) where A meets the line y=-x+3.