looks like you've got the right idea. i might modify it like this tho:

SELECT ORDER_NUM, ORDER_DATE, PART_NUM
FROM ORDERS, ORDERS_LINE
WHERE ORDERS.ORDER_NUM = ORDER_LINE.ORDER_NUM AND
ORDER_LINE.ORDER_NUM IN
(SELECT PART_NUM, DESCRIPTION, CLASS
FROM PART, ORDER_LINE
WHERE PART.PART_NUM = ORDER_LINE.PART_NUM
ORDER BY CLASS, ORDER_NUM);