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Let the speed of sound be V.    Let the distance of the point x from the left side wall be X.  Let the distance between two walls be D.

first shot is heard when the sound is reflected from the nearest wall.  Let the left wall be the nearer one.
         2 X / V = 0.6 sec  =>   X = 0.3 V
         2 (D-X) / V = 0.8 sec      =>    D- X = 0.4 V

       Hence D = 0.7 V

Now the next shot will be heard when these reflected sound waves get reflected again.  The sound wave reflected from the right wall or left wall, will totally cover 2D for that to happen.  That is initial sound makes one reflection from each of the walls once.

             So,      2 D / V = 2 * 0.7 V / V = 1.4 seconds