I hope the distance given is the horizontal distance covered = Range.
I hope also that the average speed given is = the average speed along the horizontal direction.
Then, time the disk remained in air = 139 meters / 13.0 = 10.7 seconds.
In case the given data is for a projectile in two dimensions, with an angle of projection and average speed along the path traced, the solution is not simple. It involves the path length of parabolic curve and requires integration operation etc..