I assume you are working in two dimensions so you are rotating through 180 degrees. Drop a perpendicular from (4, 3) to the line. The line, y= -x has slope -1 so the line perpendicular to it has slope 1. The line perpendicular to it and through (4, 3) has equation y= (x- 4)+ 3= x- 1. That crosses the line y= -x where y= -x= x-1 or 2x= 1 so x= 1/2, y= -1/2. To go from (4, 3) to (1/2, -1/2) x decreases by 4- 1/2= 7/2 and y decreases by 3- (-1/2)= 7/2 also. To continue on the other side by the same distance, subtract 7/2 from each again: 1/2- 7/2= -3 and -1/2- 7/2= -4.