Question 21959
If your line is (-7, 2) ; (-7, -2) then the line is a vertical line (straight up-and-down) rather than a horizontal line (flat, side-to-side).

The equation of this line would be: x = -7 and, you are right, there is no y-intercept, only an x-intercept (x = -7), so the slope-intercept form has no meaning for such a line. Since the y-coordinate of this line can take on any value you please, the coordinate of any point on this line can be written: (-7, a), where a is any real number.
A vertical line has an undefined slope (m) and no y-intercept (b) so it cannot be represented by the slope-intercept form of the equation.

Also, the equation: y = x represents a line that is at 45 degrees to the axes and passes through the origin.

Here's the graph of y=x:
{{{graph(300,200,-10,10,-10,10,x)}}}