I'm having trouble proving this one. I know its true. Any ideas? Here is what I have so far:
If $a\mid b$, then there exists an integer $q_1$ such that $b = aq_1$.
If $a\mid c$, then there exists an integer $q_2$ such that $c = aq_2$.
I know the next part is gonna be like: Therefore, $c-b=aq_2-aq_1$.
I'm just a little lost at this point.