#### Horizontal Lines

Horizontal lines have a slope of 0. Thus, in the slope-intercept equation *y* = *mx* + *b*, *m* = 0. The equation becomes *y* = *b*, where *b* is the *y*-coordinate of the *y*-intercept.

*Example 1*: Write an equation for the following line:

Graph of a Line

Since

*y* always takes the value

-1, an equation for the line is

*y* = - 1.

*Example 2*: Write an equation for the horizontal line that passes through (6, 2).

Since the line is horizontal, *y* is constant--that is, *y* always takes the same value. Since *y* takes a value of 2 at the point (6, 2), *y* always takes the value 2. Thus, the equation is *y* = 2.

#### Vertical Lines

Similarly, in the graph of a vertical line, *x* only takes one value. Thus, the equation for a vertical line is *x* = *a*, where *a* is the value that *x* takes.

*Example 3*: Write an equation for the following line:

Graph of a Line

Since

*x* always takes the value

2 = , the equation for the line is

*x* = .

*Example 4*: Write an equation for the vertical line that passes through (6, 2).

Since the line is vertical, *x* is constant--that is, *x* always takes the same value. Since *x* takes a value of 6 at the point (6, 2), *x* always takes the value 6. Thus, the equation is *x* = 6.