How about the order with invoice number 13, when was that order placed?
SELECT T1.date_order_placed FROM orders AS T1 JOIN shipments AS T2 ON T1.order_id  =  T2.order_id WHERE T2.invoice_number  =  13
order with invoice number 13 refers to WHERE T2.invoice_number = 13; when that order was placed refers to date_order_placed; joining tables orders and shipments on order_id.