P(n)=P(0)e^(kt)
If P(n)=2*P(0) (n years later population will be double of the initial one).
Then ##2=e^(k*t)##
t= years
k=population growth rate per year (which is 0.04)
Note that there is no limiting factor (or carrying capacity) in this situation.
Compute ##2=e^(k*t)##
##ln 2=t*0.04##
##0.69314718/0.04 = t##
##t=17.33 years##
Again I underline there is no upper limiting factor. Population growth is exponential.