Determine the interest rate in order to Double investment in nine years assuming interest is compounded continuously

Continuous interest is Pe^ (rt), but you are doubling your money in nine years so you want 2P=Pe^9t. Now divide both sides by P so 2=e^9t. Now take the ln of each side so ln2=9t. then just divide by 9. You will get a decimal, so move the decimal two places to the right to get your interest rate.