Yes that's correct Chris..

SELECT
COL1, Col2, Col3, First_Bal, First_Qty, S_Balance
, First_bal - S_Balance AS New_Balance
FROM (
SELECT COL1, Col2, Col3, SUM(Balance) First_Bal,
Sum(Quantity) First_Qty, 0 as S_Balance
from Table2
Group by COL1, Col2, Col3

UNION ALL
SELECT COL1, Col2, Col3, 0 First_Bal, 0 First_Qty,
SUM(MyBalance) as S_Balance
from Table1
Group by COL1, Col2, Col3
) tem