Help on Geometric Sequence Problem?

The sum of an infinite geometric series with first term
a and common ratio r < 1 is given by $S_n=a\cdot\dfrac{r^n-1}{r-1}$.
The sum of a given infinite geometric series is $S_{\infty}=200$ and the
common ratio $r$ is 0.15. What is the second term $a_2$ of this
series?

I'm confused on how to attack,can someone explain it to me? Thanks.