Let M be a plane in R n and lets assume that the plane has a null point. What is the mirror image of the point (1,2,3) in the plane M that has the form x 1 -2x 2 +x 3 = 0 ? Solution FInd the foot of the perpendicular from the point (1,2,3) to the plane x 1 -2x 2 +x 3 = 0 Hope you know how to find foot of the perpendicular, assuming it to be (d,e,f) let the image of the given point be (a,b,c) now we have (a+1)/2 = d (b+2)/2 = e (c+3)/2 = f, Knowing d,e,f values you can find a,b,c which gives you the image point .