convergence divergence - Establishing an inequality between the first term of an infinite geometric series and the infinte sum?

An infinite geometric series has the first term a and sum to infinity b, where b $\neq 0$. Prove that a lies between 0 and 2b.

$
\rightarrow \text{Since the series converges, } r \text{ has to be between 0 and 1 }\\ \text{(}\text{using the geometric series formula, i.e } \frac{a(1
- r^n)}{1 - r}\text{):}\\
\text{The sum}=b=\frac{a}{1-r}\text{, where } r \text{ is the common ratio.}\\
\rightarrow b - br = a

$

Ok. Now what? I'm stuck.