Answer
Let $a, b, c$ be integers with $a\neq 0$ and $c\neq 0$ such that $ac|bc$. $a|b$.
Work Step by Step
Let $a, b, c$ be integers as defined above. Because $ac$ factors $bc$, there exists an integer $g$ such that $acg=bc$. $a\frac{cg}{c}=b$ is an equivalent statement by the properties of integers, and $\frac{cg}{c}=g$ is an integer for the same reason. Because $g$ is an integer s.t. $ag=b$, $a|b$. This proves the theorem above.
