The radius of the tire = r = 0.4m
We want to find how many radians the tire turned while rolling down the street. We know the distance that the tire moved and its radius, so by dividing the distance by the radius (you could also find the number of revolutions the tire makes and then change that number to radians, but the
would cancel out) you find the radians moved by the tire. Using 
and the initial rotational velocity you can now solve for the average rotational acceleration.
