The equation of the right bisecting plane of line segment joining the points A≡(a,a,2a)  and B≡(−a,−a,a)  is

Mid-point of AB=(0,0,3a/2)  and D.R.’s of AB are (2a,2a,a) ∴  Equation of right bisecting plane of AB  is 2a(x−0)+2a(y−0)+a(z−3a/2)=0 ⇒4x+4y+2z=3a .