To find the rate, first you divide 180mi/3hr to get 60mph. On the first day they were traveling at a rate of 60mph. It says that on the second day, they drove at the same rate and drove 300 miles. This means you divide 300mi/60mph to get 5hr. They drove 5hr on the second day. And finally, the question asks how long they drove total, so you add 3hr+5hr to get 8hr total.
Answer is 8hr.