Help with exponential problem.

I could use some help understanding these problems and setting up the equation using the formulas.

problem:

A single-celled amoeba doubles every 4 days. How long would it take one amoeba to produce a population of about 10,000 amoebae?

I am a little confused about which formula to use.

FV= R ((1+i)^n)-1/i

or A=R(1+r)^n

or A=R(1+r/k)^kt

I thought to use the third one, making the equation A=10,000(1+2/4)4t then solve from there.

Is that formula correct or even the right way to set up that formula?

thanks a lot.