Solomon six shooter fires a gun if the bullet leaves the barrel of the gun with velocity of 500 meters per second how long will it take to travel 2000 meters?

That's going to depend a lot on what direction he points the gun when he fires it.

For example, if he's holding it level and fires parallel to the ground, then
it would travel 2,000 meters in (2000/500) = 4 seconds after the shot. 
BUT . . . in order to accomplish that, the muzzle would have to be at least
78 meters (about 255 feet) up off the ground, because anything that's being
attracted by gravity falls about 255 feet in 4 seconds, and if the bullet hit the
ground sooner than that, then it would stop traveling horizontally.

If he points the gun straight up, then the bullet reaches 2,000 feet above
the muzzle in 4.17 seconds after the shot.  It keeps going up, tops out when
it runs out of speed, starts falling, and returns to 2,000 feet again at about
98 seconds after the shot.  (At 102 seconds after the shot, it is hoped that
Solomon has moved out of the way, because that's when the bullet returns
to the ground, at the same speed of 500 m/s that it had when it was fired
straight up.  That speed is a little over 1,100 mph, believe it or not, and that
little tiny piece of lead falling out of the sky is dangerous.  Solomon would
not be wise to stand there and look up.)