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