select Customers.cust_name,Orders.order_num,tb.OrderTotal 
from Customers 
inner join Orders 
on Customers.cust_id=Orders.cust_id inner  join 
(select order_num,sum(quantity*item_price) OrderTotal from OrderItems group by order_num) tb
on Orders.order_num=tb.order_num
order by cust_name,order_num;