First thing the formula is slightly mistated.
it should be
A=P(1+r/n)nt (you had rt fo the exponent and it should be nt
now lets solve this thing
you want A = 2P (double your money
2P = P(1+r/n)nt
we can divide through by P adn put in the values we know
2 = (1+.085/2)2t
2 = (1.0425)2t
and now we can take the log of both sides to make it easy to solve our equation
log(2) = 2tlog(1.0425)
.301 = 2t (.018)
divide both sides by .018
16.653 = 2t
t = 8.327 (number of years to double your money at this rate
check it
(1.0425)16.653 =2
