The tires of a car make 65 revolutions as the car reduces its speed uniformly from 100 km/h to 50 km/h. The tires have diameter of 0.80 m. a) What was the angular acceleration? b)If the car continues to decelerate at this rate, how much more timeis required for it to stop?

For part a, I thought my method was pretty legitimate, but myanswer didn't match the one in the back of the book. The backof the book had the answer -4.4 $ra\frac{d}{{s}^{2}}$, whereas Igot $-1.59\cdot {10}^{4}$ $ra\frac{d}{{s}^{2}}$