All you are trying to do here is to find what you get by investing at compound interest. The formula for compound interest is
A = P(1+r/n)^nt
In this formula, A is the amount of money you have after t years. P is the original amount of money invested, r is the annual interest rate expressed as a decimal and n is the number of times interest is compounded each year.
So now plug in the numbers that you have provided.
A = 5000(1+.066/12)^12*6
If you do the math correctly you will come out with $7421.29. So your original $5000 investment will become $7421.29, which means that you earned $2421.29 in interest during the 6 years.