So I am not really sure what to do. I know by the definition of divisibility there must exist some integers $k$ and $l$ such that
$$ c= ak \ \text{ and } \ c=bl $$
But now I am stuck and have no clue where to go from here...
I need to show that $c=ab(\text{some integer})$, for it to be divisible, but I do not see the path to take.
If $a= b = c > 1$ then $a|c, b|c, ab \not\mid c$.