If $A(2\overline{i}+3\overline{j}-6\overline{k}), B(6\overline{i}-3\overline{j}+2\overline{k})$ then the perpendicular bisector of $\overline{AB}$ through O is 

OA=OB=7$\therefore$$∆$OAB is an isosceles triangle 

Mid point of AB=$\overline{OM}$ =$\frac{8\overline{i}-4\overline{k}}{2}$
$\begin{array}{l}=4\overline{i}-2\overline{k}\\ \therefore \overline{r}=t(4\overline{i}-2\overline{k})\text{\hspace{0.17em}is\hspace{0.17em}\hspace{0.17em}the\hspace{0.17em}\hspace{0.17em}perpendicular \hspace{0.17em}bisector}\end{array}$