Solution
For the flip-flop to operate properly, there must be a PT on the EN input. While EN is high, the information on R and S caused the latch to set or reset. Then when EN transitions back to low, this information is retained in the latch. When this NT occurred, both R and S inputs were low (0), and thus there was no change of state. In other words, the value of Q at tine n + 1 is the same as it was at time n. Remember that time n occurs just before the NT on EN, and time n + 1, occurs just after the transition.
The logic diagrams shown in Fig. 8-12a and b illustrate two different methods for realizing a clocked RS flip-flop. Both realizations are widely used in medium- and large scale integrated circuits, and you will find them easy to recognize. You might like to examine the logic diagrams for and 54LS109 or a 54LS74, for instance.