with specific formula and explaination plz

24. Tom has served in the Navy for 21 years and plans to retire sometime in the future. Every three years he has received a re-enlistment bonus and deposited a certain dollar amount, X, in an account for when he retires. After making his current deposit, the value of his accqunt is $180,176. Assuming an annual effec- tive interest of 9%, what were his tri-annual deposits? Ansuwer: X =10,405.00