The series is given as
$$\sum_{k=2}^{\infty}\left(\frac{3}{8}\right)^{3 k}$$
This is a geometric series where the first term (a) is $\left(\frac{3}{8}\right)^{3 \cdot 2} = \left(\frac{3}{8}\right)^6$ and the common ratio (r) is $\left(\frac{3}{8}\right)^3$.
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