To find the unit rate, all we have to do is calculate the actual division. On the first item we have "$156 for 3 books".
This rate tells us the price every 3 books we buy. The unit rate would be the price of 1 book.
We have a unit rate of $52/book.
In the second item, we have "250 miles in 4 hours"
We have a unit rate of 62.5 miles/h.