What is the equation (y=mx+b) of the horizontal line through point a and the equation of the vertical line through point a?

A horizontal line is simply y = the y-value of the line. This is because the line has a slope of 0, making y = mx + b = 0x + b = b, where m is the slope and b is the y-intercept. Thus, the horizontal line has the equation y = -6

A vertical line has does not fit the equation y = mx + b because the line is not a function due it having one x-value with multiple y-values. However, this line can be represented as x = x-value of line. Thus, the equation of the vertical line through point A would be x = 4.