My bad, I can't do it in one line.
Q \rightarrow P: $As I said above, use that if$ \;a|b\; $and$ \;a|c\; $then for some integers$ \;s\; $and$ \;t,
$we conclude$ \;a|sb+tc. \;$Prove of this is I don't think necessary but$ \;7|x \;$and$\; 7|y \;$implies$ \;7k=x, 7m = y. \;$Multiplying$\; (1)\...