Answer
$x=0$
Work Step by Step
The $y$-axis is a vertical line and therefore should have an equation with a form of $x=a$, where $a$ is a constant and is actually the $x$-coordinate of all the points on the line.
The origin $(0,0)$ is on the line, therefore here the $x$-coordinate equals to $0$.
As the origin is on the line, if we plug in $x=0$ this should satisfy the equation. Therefore the equation itself is $x=0$.
