Hi there! I'm having some big trouble with this:

Let there be $\displaystyle x, y \in \mathcal{Z}$. Show that $\displaystyle 10x + y$ is a multiple of $\displaystyle 7$ if, and only if $\displaystyle x - 2y$ is a multiple of 7.Using the equivalence above, find a divisibility criteria for $\displaystyle 7$.

So, I'd love to show you where I'm stuck, if it was not in the very beginning. Tips?