Horizontal Lines
From the section on slope, we have seen that the slope of a horizontal line is zero. Look at the graph of the horizontal line below:
If we list a few of the points that are on the horizontal line shown above such as (-4, 2), (-2, 2), (0, 2), (2, 2), (4, 2), etc., we see that every point has a y-coordinate of 2.
The equation 
completely describes the line, because y is 2 for any value of x. Because this line is horizontal, the value of x can be any number, but the value of y is always 2.
The equation of any horizontal line (whose slope is zero) is 
, where b is the y-coordinate of every point on the line.
Example 10. Write the equation of the line shown above:
Answer:
The equation is 
because every point on the line has a y-coordinate of -3.
Example 11. Write the equation of the line which has slope of zero and passes through the point (3, 16).
Answer:
A slope of zero means that the line is horizontal. The line has the equation 
because it must pass through the point (3, 16).