Compound Interest problem..please help..

Hello guys..please help me on this..I've been doing this question for 3 days..ahahaha!

A tin mine is estimated to have tin reserves worth two million dollars. If the monthly extraction is 5% of the value of the reserve the month before, find the value of the tin reserves after two years.

I know if I use the geometric progression, I'll get the answer somewhere around 583978.05 and I guess that's the right answer but I have to use compound interest formula..I think maybe it's present value formula...but I'm totally lost..please help...