1. The vectors $a$, $b$ and $c$ are given by

$a=(\begin{matrix}3\\5\end{matrix}),b=(\begin{matrix}-3k\\k\end{matrix}),c=(\begin{matrix}0\\-4\end{matrix})$

where $k$ is a constant.

Given that $a-b$ is parallel to $a+c$, find the value of $k$.