Please please help, if you could show work too that would be amazing! :)

Acircle has a diameter will endpoints of (-2,8) and (6, 4). what is the center of the circle

Fanny mendelssohn manages all the magazines and paperback books at a local grocery store. she prices many paperbacks to sell at $7.99. the cost to the store is $5.29. what is the markup rate on a paperback book, based on its cost?

Can someone me with #s 8, 9, and 11. with just one of the three also works. prove using only trig identities.