The distance traveled by the ball is given as 139 m and the average speed of the ball (through the air) is 13 m/s. Speed is defined as the ratio of distance traveled to time taken. In other words,
or, time = distance/speed
Thus, the time spent by the ball in air can be calculated as the ratio of distance and average speed.
or, time spent by the ball in air = `(139 m) / (13 m/s)` =10.69 s
Thus, the ball spent 10.69 s in air for its travel from wide receiver to his girlfriend in the stands.
A number of other calculations can be done on similar problems by using equation of trajectory and related equations of motion of objects. For example, we can calculate the maximum height attained by the ball as it was thrown from one player to the other.
Hope this helps.