Solve the equation $3x - 1 = x + 7$ and represent the solution$(s)$ on $(1)$ the number line $(2)$ the Cartesian plane.

(N/A) Step $1$: Solve the equation for $x$.
$3x - 1 = x + 7$
$3x - x = 7 + 1$
$2x = 8$
$x = 4$
Step $2$: Representation on the number line.
On the number line,mark the point $4$ as a solid dot.
Step $3$: Representation on the Cartesian plane.
In the Cartesian plane,the equation $x = 4$ represents a vertical line passing through $x = 4$ on the $x$-axis,parallel to the $y$-axis.