A trucker drove 9 hours to deliver a shipment of lumber. Returing home with a lighter load, the trucker averaged 8 miles per hour faster than on the earlier trip. After 7 hours, the trucker was only 60 miles from the starting point. The equation 9 s equals 7 open parentheses s plus 8 close parentheses plus 60 models the situation, where s represents the speed traveling to the delivery site. What is the value of s, in miles per hour? ( explain your answer)