A line can be written in slope-intercept form, which is:
where m represents the slope and b the y-intercept.
If we compare the third equation
with the slope-intercept form, we have the slope and y-intercept:
The slope of a line is a measure of its steepness. The lines:
are both horizontal lines, therefore, the slope is undefined for both of them. The y-intercept is the point where the graph crosses the y-axis. One of the lines is the y-axis, therefore, it "cuts" the y-axis on every single point. The other line is just parallel to the y-axis, therefore, there's no y-intercept.
![\begin{gathered} x=0\implies\begin{cases}m={undefined} \\ b={y}\end{cases} \\ x=-15\implies\begin{cases}m={undefined} \\ b\in{\varnothing}\end{cases} \end{gathered}]()