Speed is in the units of distance divided by time. You gave us a distance measured in feet and a time in seconds so your speed in feet per second is
500/6 = 83.3 feet per second.
I expect however that you want the speed in miles per hour. There are 5280 feet in a mile so
500 ft = 500/5280 miles.
There are 60 seconds in a minute and 60 minutes in an hour and hence there are 60 60= 3600 seconds in an hour. Thus your time of is
6 seconds = 6/3600 hours
To find your speed in miles per hour divide the number of miles by the number of hours.