Remember, if you have trouble finding the command for a math symbol you want to use, try googling it, email me, or try looking in this Short Math Guide for $\LaTeX{}$.
Suppose that $G$ is a group, and let $g\in G$ with $|g| = n$. Prove that if $g^i = g^j$, then $n$ divides $i-j$. (See Theorem 4.24, but I’m only asking you to prove one direction of the “if and only if”.)
Prove that if $G$ is an infinite cyclic group, then $G$ is isomorphic to $(\mathbb{Z},+)$. (See Theorem 4.29.)