I have often heard that we should exercise daily. so can i exercise 6 days a week (3 cardio days, 3 weight lifting) and rest on sundays?

Yes, u can...: Really, talking about two different things...@ age 36 you probably would want to work out aerobically ( run , bike, swim , elliptical , tennis etc.)5 days a week for ~ 30 min a session.( w/ goal of reaching 70-80% of max heart rate ).This is standard aerobic ( heart/ lung) workout( see your pcp first before starting). Weights are for toning and building muscle primarily; cv workout here is <<.Gl!

That is great: The current recommendations for exercise are for aerobic exercise, totaling more than 30 minutes a day, on "most days of the week". Most physicians thus interpret that as >4 days a week. So if you are exercising 6 days a week, that is great, and i commend you for that. That extra rest day sounds perfectly fine if you are exercising on most of the other days of the week.