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