To know if (3,20) is a solution to the given system, we just have to evaluate it.
[tex]\begin{cases}y=3x-7 \\ y=4x+8\end{cases}\rightarrow\begin{cases}20=3\cdot3+7 \\ 20=4\cdot3+8\end{cases}[/tex]
Let's solve it
[tex]\begin{cases}20=9+7 \\ 20=12+8\end{cases}\rightarrow\begin{cases}20=16 \\ 20=20\end{cases}[/tex]
As you can observe, (3,20) satisfies just one equation. Hence, it's not a solution to the system.
