The given power series is
$$\sum_{k=0}^{\infty}(-1)^{k} \frac{x^{3 k}}{27^{k}}$$
We can rewrite this as
$$\sum_{k=0}^{\infty}(-1)^{k} \left(\frac{x^{3}}{27}\right)^{k}$$
This is a geometric series with common ratio $r = -\frac{x^{3}}{27}$.
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