What is the average turnaround time?
You need to determine at what time each job is completed. With a first-come-first-served scheduler, this is simple to calculate: each job starts as soon as the processor becomes free, and takes exactly its burst time to complete. You've already calculated the start and end times to calculate the wait times, so use that to obtain the turnaround time.

For example, A arrives at time 0. The processor is free, so it starts at time 0 and ends at time 6. Then the processor runs B, which had to wait for 5 units, and finishes at time 8, for a turnaround time of 7.

The answer from the book seems to be totaling the completion times, without regard for the arrival time. This is not something I recognize as “turnaround time”.