The reflection of the point a→ in the plane r→⋅n→=q is

Given plane is  r→⋅n→=q---iLet the image of A(a→) in the plane be Bb→Equation of AC is r→=a→+λn→(∵AC is normal to the plane) ----iiSolving (i) and (ii), we get (a→+λn→)⋅n→=qor  λ=q−a→⋅n→|n→|2∴  OC→=a→+(q−a→⋅n→)a→2⋅n→but   OC→=a→+b→2∴  a→+(q−a→⋅n→)n→|n→|2=a→+b→2or   b→=a→+2q−a→⋅n→|n→|2n→