Question: What is the total amount of tax charged for the order placed by customer 88931 on 7/13/994?
SQL: SELECT SUM(T2.l_extendedprice * (1 - T2.l_discount) * (1 + T2.l_tax)) FROM orders AS T1 INNER JOIN lineitem AS T2 ON T1.o_orderkey = T2.l_orderkey WHERE T1.o_custkey = 88931 AND T1.o_orderdate = '1994-07-13'