"Use the Ratio Test to determine the values of x greater than or equals $x ≥ 0$ for which the series converges." $$\sum_{k=1}^\infty \frac{3x^k}{k^3}$$
I'm having trouble with the x variable. After setting up the ratio and canceling out terms I've gotten to this point but now i'm stuck:$$\frac{xk^3}{(k+1)^3}$$ The answer is has the series converging for $x<1$ but i'm not understanding how.
$$\frac{xk^3}{(k+1)^3}=\frac{x}{(1+\frac{1}{k})^3}$$