A machine part has the shape of a solid uniform sphere of mass 250 g and a diameter of 4.30 cm. It is spinning about a frictionless axle through its center, but at one point on its equator it is scraping against metal, resulting in a friction force of 0.0200 N at that point.

PART (A):

Find its angular acceleration. Let the direction the sphere is spinning be the positive sense of rotation.

PART (B):
How long will it take to decrease its rotational speed by 21.0 Rad/s?

NOTE: For part (A), I've tried solving using a similer question, and I got Angular Accelration = 6.59 Rad/s^2 which is wrong, according to Mastering Physics. For Part (B) I got 6.5 seconds, which is also wrong. I don't understand what I'm doing wrong since I followed the exact same method used in the same question posted on chegg.