Two circles with centre A and B and of radii 5 cm and 3 cm touch each other internally.If the perpendicular bisector of segment AB meets the bigger circle in P and Q,then the length of PQ is ___________

Question

$\begin{array}{1 1}24\;cm\\4\sqrt 6\;cm\\3\sqrt 6\;cm\\18\;cm\end{array}$