\( \displaystyle\sum_{n=2}^\infty{2^n-1\over5^{n+1}} = \dfrac{2^2}{5^3}\displaystyle\sum_{n=0}^\infty{\left(\frac{2}{5}\right)^n } - \dfrac{1}{5^3} \displaystyle\sum_{n=0}^{\infty}\left(\dfrac{1}{5}\right)^n \)
Y la suma de esas series las debés conocer.