The line x=7 is a vertical line that passes through the point (7, 0). The slope of all vertical lines is undefined. Since the graph of x=7 will not intersect the y-axis. The y-intercept does not exist.
When the equation of a line is given as x=1, x=-12, x=2/3, .... it ALWAYS means that the slope it's ALWAYS UNDEFINED (in Calculus you'll learn what this means). So, undefined means NO SLOPE, not zero, zero it's a slope, these equations' slopes are UNDEFINED.
Since x=7 doesn't have a "y" there
the equation x=7 is parallel to x= 0 , as both are vertical lines so the slope is `oo` .
and the y-intercept does not exist.