Plotting lines parallel to x and y axis

When one variable equals a constant, the graph is a line parallel to an axis.
$y = c$ is horizontal, parallel to the $x$-axis.
$x = k$ is vertical, parallel to the $y$-axis.

The figure shows both kinds.
The horizontal line $y = c$ keeps $y$ fixed while $x$ varies, so it runs left-to-right parallel to the $x$-axis.
The vertical line $x = k$ keeps $x$ fixed while $y$ varies, so it runs up-and-down parallel to the $y$-axis.

A horizontal line $y = c$ crosses the $y$-axis at $c$.
A vertical line $x = k$ crosses the $x$-axis at $k$.
So $y = 5$ passes through $(0, 5)$, and $x = -2$ passes through $(-2, 0)$.

Sometimes an equation needs rearranging first.
$3x = a$ becomes $x = \frac{a}{3}$, which is a vertical line, since $x$ equals a constant.
Any equation where a single variable equals a constant is a line parallel to an axis.

What does the line $y = 5$ look like?
It is a horizontal line, parallel to the $x$-axis.
Every point on it has $y = 5$, so it crosses the $y$-axis at $(0, 5)$.

What does the line $x = -2$ look like?
It is a vertical line, parallel to the $y$-axis.
Every point on it has $x = -2$, so it crosses the $x$-axis at $(-2, 0)$.

What kind of line is $2y = 6$?
Rewrite it as $y = 3$, which sets $y$ equal to a constant.
So it is a horizontal line, parallel to the $x$-axis.