How can I index or make this code faster. The sum query is the slow point.

[Nathan Stanford]

SELECT a.ORD_ID AS Manual_Added_Orders,
a.ORD_poOrdID_List AS Auto_Added_Orders,
a.ORDPOITEM_ModelNumber,
a.ORDPO_Number,
a.ORDPOITEM_ID,
(SELECT sum(ORDPOITEM_Qty) AS ORDPOITEM_Qty
FROM orderpoitems
WHERE ORDPOITEM_ModelNumber = a.ORDPOITEM_ModelNumber
AND ORDPO_Number = 123007)
AS ORDPOITEM_Qty,
a.ORDPO_TrackingNumber,
a.ORDPOITEM_Received,
a.ORDPOITEM_ReceivedQty,
a.ORDPOITEM_ReceivedBy,
b.ORDPO_ID
FROM orderpoitems a
LEFT JOIN orderpo b ON (a.ORDPO_Number = b.ORDPO_Number)
WHERE a.ORDPO_Number = 123007
GROUP BY a.ORDPOITEM_ModelNumber,a.ORD_poOrdID_List, a.ORD_ID
ORDER BY a.ORDPOITEM_ModelNumber,a.ORD_poOrdID_List, a.ORD_ID


I did an Explain but I am not sure what to do to make a good index or how to make the sum go faster.