For a non empty set $S$ of a group $G$,define $$S^k=\{\Pi_{i=1} ^{n} s_i | s_i \in S\}$$ for any positive integer $k$. Prove that, if $G$ has $n$ elements, then $S^n$ is a subgroup of $G$.
I am a beginner in abstract algebra so couldn’t achieve something that I could discuss