Answer
The angle through which each wheel rotates is $20.65$ radians.
Work Step by Step
From the figure, we see that the axle rolls down in a circular path. The distance the axle travels is the arc length subtended in an angle $\theta=0.96rad$ at a radius $r=9-0.4=8.6m$. In other words, $$s_{axle}=r\theta=8.26m$$
In rolling motion, the distance the axle travels is similar to the distance each wheel has traveled. Therefore, $s_{axle}=s_{wheel}=8.26m$
The wheel's radius $r=0.4m$. The angle through which each wheel rotates is $$\theta=\frac{s_{wheel}}{r}=20.65rad$$
