Answer
It takes 0.0758 seconds between the sound reaching the ear directly, and the sound reaching the ear after reflecting off the back wall.
Work Step by Step
If the person is sitting in the middle row, then the person is 13.0 meters from the stage and 13.0 meters from the back wall. After the sound from the stage reaches the person directly, the sound must then travel an additional 13.0 meters to the back wall and then 13.0 meters back to the person, for a total of 26.0 meters. We can find the time it takes for the sound to travel an additional 26.0 meters.
$t = \frac{d}{v}$
$t = \frac{26.0~m}{343~m/s}$
$t = 0.0758~s$
It takes 0.0758 seconds between the sound reaching the ear directly, and the sound reaching the ear after reflecting off the back wall.
