Question

Find the equation for the horizontal line that goes through point M(a, b).

Answer 1

Horizontal means zero slope, or a zero coefficient on x.

Answer: y = b

Answer 2

Remark

The line is horizontal. That means that the equation is y = something. There is no x value for this. If it goes through M(a,b) then what you are concerned about is b

y = b is the equation of the line you seek.