You are referring to the reflection of the complex number z= x+ iy (or point (x,y)) in the line y= kx?

I don't know any formula off the top of my head but you should be able to work it out. The "reflection" of a point in a line is the point, on the opposite side of the line, such that the line connecting them is perpendicular to the given line and the distance from each point to the given line is the same. The line perpendicular to y= kx has slope -1/k so the line through perpendicular to y= kx is . That will cross y= kx when . Then so that . Since this point is on y= kx, [tex]y= \frac{k(x_0+ ky_0)}{k^2+ 1}. You can calculate the change in x from to as well as the change in y from to and add those again to the point to get the corresponding point on the other side of the line.