Answer
$y=0$
Work Step by Step
If we are searching for the equation of the $x$-axis, we have to note, that it is a horizontal line.
Therefore the equation will be of the form of $y=a$, where $a$ is a constant.
The $x$-axis passes by every point that has a $y$-coordinate of $0$.
Hence, the equation of the $x$-axis is: $y=0$.
