+118723 $$\begin{array}{rlll}
3439.96&=&3104.31(1+r/4)^{20}\\\\
3439.96/3104.31&=&(1+r/4)^{20}\\\\
(3439.96/3104.31)^{1/20}&=&1+r/4\\\\
(3439.96/3104.31)^{1/20}-1&=&r/4\\\\
4[(3439.96/3104.31)^{1/20}-1]&=&r\\\\
\end{array}$$
You can finish it on the site calc on the home page. 
