Sometimes the trouble people have with these is not knowing how to set up "doubling" into the equation, but it's actually quite simple. You could literally use any numbers you like, as long as your future value in 15 years is twice what you start with. And 1 for today and 2 for 15 years from now works just as well as anything, and is easier cause 1 is always an easy number. :-)
I'm going to assume you must use the equation to solve for this.
future amt = present amt (1 + r)n
So 2 = 1 (1 + r)15 where you are solving for r.
You also may be having trouble with the solving part of that. I'll do an example using tripling in 20 years:
3 = 1 (1 + r)20
First we can just drop that 1, since 1 times anything is itself. If it helps to think of it like this, then divide both sides by 1 to get rid of it. The 1 will be gone from the right and the left will still be 3 anyway.
3 = (1 + r)20
Now we get to the tricky part. We need to deal with the exponent. To eliminate an exponent, you need to take the nth root of both sides, in this case the 20th root. The 20th root of a 20th exponent will rid you of that exponent, leaving only 1 + r:
20√3 = 20 √[(1 + r)20]
(You'll have to deal with how to get 20th root of 3 on your particular calculator.)
1.05646... = 1 + r
Now you can just subtract 1 from both sides and you're left with:
.05646 = r
Or r = ~5.6%