Mu$$strame c$$mo resolver Calculating deposit needed$$You put $$$$9,000$ in an account earning $4\%.$ After $4$years$,$ you make another deposit into the same account.$$Three years later$($that is$,7$ years after your original $$$$9$ comma $000$deposit$),$ the account balance is $$$$21,000.$

$-$What was the amount of the deposit at the end of year $4?$