Not sure where to go from here
No need to expand.
From $\displaystyle (x-3)^3 = 125$, we get $\displaystyle x-3 = 5 \Rightarrow x = 8$. There are also two roots in the complex plane, $\displaystyle x = 3 + 5e^{\frac{2\pi i}{3}}$ and $\displaystyle x = 3 + 5e^{\frac{4\pi i}{3}}$