#### Answer

$y=0$.

#### Work Step by Step

RECALL:
The graph of the equation $y=k$ is a horizontal line whose y-intercept is $(0, k)$. Every point on the line has a y-coordinate of $k$.
Note that the x-axis is horizontal, and every point on the x-axis has a y-coordinate of $0$.
Thus, the equation of the line that lies along the x-axis is $y=0$.