Question 21959
You are right that the line (-7,2) ; (-7,-2) is a horizontal line which can be stated as (y=x).

Using the formula y=mx+b,  the slope "m" is 0, since it is a horizontal line and  "b" is where it intersects the y-axis, in this case, at -7.

Therefore, the equation of a horizontal line at -7, is y=x-7.

All horizontal lines will be written this way, for example, y=x+5, if it is at +5. y=x is basically the x-axis at y=0, so you can go up or down on the y-axis by adding or subtracting to that basic equation.