Hi,
initial investment of $5,000 doubles in value every 6 years: (A/P = 2)
In General 
A = Accumulated Amount
P= principal(Initial Investment) =
r= annual rate = .
n= periods per year = 1
t= years =

e^(ln2/6)= 1+r
.1225 = r, 12.25% Simple Interest/yr
A = P(1.225)^t
How much will the investment be worth in 18 years
6 yrs = $10,000
12 yrs = $20,000
18 yrs = $40,000