In order to complete one complete oscillation, the bob of the pendulum will move from R to P, P to R, R to Q and then Q to R.
The time taken by the bob of a pendulum to move from point R to P is 3 s.
It takes equal time to move from R to P, P to R, R to Q, and Q to R, i.e, 3 s.
So, total time taken by the pendulum to complete one oscillation
= (3+3+3+3) = 12 s