Brushing Up on Set Theory Struggling with a simple proof: Let $X = \{3^n\mid n > 0\}$ and $Y = \{3n\mid n \geq 0\}$. Prove that $X$ is a subset of $Y$.
I tried Direct Approach and Contrapositive, but I'm not strong enough with my proof skills yet. Can someone show me some steps to doing this? I don't need a direct solution, just some steps to get me in the right direction so I have a good example.

You need to show that if $x$ is an element of $X$, then $x$ is an element of $Y$ as well. Namely, if $x=3^n$, for some $n\in\Bbb N^+$, then there is some $k\in\Bbb N^+$ such that $x=3k$ (where $\Bbb N^+$ denotes the non-zero elements of $\Bbb N$, of course).
Can you think what $k$ should be?
