Thor invests $750 in an account, with interest compounded continuously. if his investment doubles in value after 9 years, how much money will he have after 12 years? [use the formla a = pert and round answer to the nearest cent.]

We'll first solve for the rate:
This formula applies to continuously compounded interest.
After 9 years, his $750.00 will become 1,500.00
rate = ln(Total / Principal) / years
rate = ln(1,500/750) / 9
rate = ln(2) / 9
rate = 0.69314718056 / 9
rate = 7.70163534 compounded continuously

Now, here's the continuously compounded interest formula for total.
Total = Principal * e^(rate*years)
Total = 750 * 2.718281828^(.0770163534 * 12)
Total = 750 * 2.718281828 ^ ( 0.9241962408)
Total = 750 * 2.5198420995
Total = 1,889.88 after 12 years

You can find those formulas here: http://www.1728.org/rate2.htm
And there's a compound interest calculator here:
http://www.1728.org/compint.htm