That's great, thank you. Then could you please let me know what is the sum of those two accounts?
SELECT T2.balance + T3.balance  FROM accounts AS T1 JOIN checking AS T2 ON T1.custid   =   T2.custid JOIN savings AS T3 ON T1.custid   =   T3.custid WHERE T3.balance  >   ( SELECT avg ( balance )  FROM savings ) 
sum of two accounts refers to T2.balance + T3.balance; comparing to an average balance involves a subquery to calculate avg(balance) from a table; joining tables based on a common identifier, in this case, custid;