how to cook a 6 pound prime rib to med rare

Pick your target temperature and insert a remote read thermomator.

Low and slow.

Get it up to about 125-130 degrees and pull it from the oven because it will continue to "cook" while it's resting.

Look here at the recipe for classic rib roast.
Or at other sites you respect which give full instructions and understanding, like Serious Eats.

i mean how long to roast it i should have said..........THANK YOU!!