How much time will elapse in seconds before you see explosion 5400 feet away? assume speed of light is 343 m/s

The speed of light is NOT 343 m/s.  That's the speed of sound.
You HEAR the explosion after

       (5,400 ft) x (1 meter / 3.28 ft) x (1 sec / 343 meters) =  4.80 seconds.

The speed of light is 299,792,458 m/s.
You SEE the explosion after

       (5,400 ft) x (1 meter / 3.28 ft) x (1 sec / 299,792,458 meters) =

                                                                                     0.0000055 second .

The rearrangement of particles in a physical change. hope this