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 .

Ihave no idea sorry

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.