If z_1, z_2, z_3 ∈ C are the vertices of an equilateral triangle, whose centroid is z_0, then Σ_k=1^3(z_k-z_0)^2 is equal to

If $\displaystyle z_1, z_2, z_3 \in \mathbb{C}$ are the vertices of an equilateral triangle, whose centroid is $\displaystyle z_0$, then $\displaystyle \sum_{k=1}^3\left(z_k-z_0\right)^2$ is equal to