how long do I cook a 6.6 pound beef tenderloin in a 375 oven for medium rare?

I'd go a bit lower than 375. First, sear it on all sides and ends either on a bbq grill or around 375-400 in the oven for a few minutes on each side. Then reduce the heat to 300 so that it roasts gently. Remove it from the oven when a thermometer reads 118-120, cover it with foil and a couple of heavy kitchen towels and let it rest for 20 minutes before carving. The rest period allows juices migrating around outside cell walls (migrating around because you roasted the meat at a gentle temperature) to re-inflate the cells, allowing for a more tender, moist roast. Enjoy your dinner!

Start checking at 40 minutes. For medium rare an thermometer should read 130. It could take up to an hour