The circumference of the wheel is $2\pi \cdot 12 in$. The wheel must turn $\frac{57 \cdot 63360 in}{2\pi \cdot 12 in}$ times to travel the distance of $57 mi = 57 \cdot 63360 in$. Each full rotation turns the wheel (and thus the nail) by an angle of $2\pi$. This happens in an hour, so the angular velocity of the nail is $2\pi \frac{57 \cdot 63360 in}{2\pi \cdot 12 in}/h = 300960/h$.