Physics help?

The question is: In 1987, a giant hanging basket of flowerswith a m of 4000 kg was constructed. The radius of the basket was 3.0 m. Suppose this basket was placed on the ground and an admiring spectator ran around it to see every detail again and again. At first the spectators's angular speed was 0.820 rad\s but he steadily decreased his speed to 0.360 rad\s by the time he had traveled 20.0 m around the basket. Find the spectator's angular acceleration...help please!!!!