P = $500 the principal

r = 0.06 or 6% annual interest rate

A = $991.99 the future value (double the principal)

t = the time in years

A = P(1 + r)^t

Plug the values into the last equation:

991.99 = 500(1 + 0.06)^t

500(1 + 0.06)^t = 991.99

1.06^t = 991.99/500

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t = 11.76 yr

It will take 11.76 years for $500 to grow to $991.99.