Originally Posted by

**choovuck** I won't actually solve you the question, but it shouldn't be hard. the light travels along the vector (8,4,1) initially, and then along the vector (2,4,4) (subtract the coordinates). therefore the normal to the mirror vector v is precisely the bisector vector of (-8,-4,-1) and (2,4,4). Finding the bisector vector of two vectors should be easy/standard, and after that it's easy to write the equation of the plane passing through the point (8,4,1) with the normal vector v