...Yeah, homework help.

I've been out of my College Algebra class for two weeks with mono, strep, and jaundice (oh boy!). I've never well understood compound interest, but I think I get it from teaching myself. I'm trying to get a headstart on making up my work, and the paper asks:

What is the interest rate of an account that doubles your money in seven yeas when interest is compunded quarterly?

My answer is some horrible thing. I'll describe it:
4(Twenty-eighth root of the principal)-4=r

Is this correct? Would anyone like to confirm this or help me?
Thanks.