I am trying to prove that $\epsilon_{ijk}$$\epsilon_{lmn}$ equals to the determinant on the right. I calculated the determinant and write out all the non-zero sum of $\epsilon_{ijk}$, but then I don't know what to do next. I am also confused about what $\epsilon_{ijk}$$\epsilon_{lmn}$ means. I thought it was a summation, but other people pointed out that it isn't. So if anyone can explain it with simple concepts would be appreciated.
note: $\epsilon$ is the anti-symmetric rank 3 tensor and $\delta$ is the Kronecker delta