As a very rough rule of thumb, we can use the "rule of 70".
At 9 percent interest, we can estimate that the money will double in approximately 8 years (8*9=72).
Therefore, your answer should be close to $4700.
The rule of 70 is a way to ESTIMATE how long it will take an investment to double at a given interest rate.
7% * 10 years
8% * 9 years
9% * 8 years
12 % * 6 years
14 % * 5 years
etc...