The x-axis is like a number line, isn't it? Every point on the x-axis has a y-coordinate of 0 like this: (-4,0), (3,0), (2.7, 0) and (0,0).
If all of these points have the same y-coordinate, it follows that the equation of that line must be y = 0! It would be the same idea for any horizontal line, since the slope = 0. Calculate the slope between any two of those points:
m = #(0-0)/(3-(-4))# using (-4,0) and (3,0).
You would write the equation now like: y = 0x + 0, or just y = 0.
Think about another horizontal line that goes through the points (8,3), (0,3), (-14, 3), and (4.1, 3). Calculate the slope:
m =#(3-3)/(-14-0)# using the points (0,3) and (-14,3).
The y-intercept is (0,3) and therefore the equation of that line is y = 0x + 3, or just y = 3.