We will have that from these systems the ones that won't have a solution are the ones that are a set of parallel lines that don't have any point in common, that is:
[tex]\begin{cases}y=5-3x \\ \\ y=-3x+4\end{cases}[/tex][tex]\begin{cases}3x+7y=42 \\ \\ 6x+14y=50\end{cases}[/tex]