Beginning Algebra With Applications, Chapter 5, 5.4, Section 5.4, Problem 58

Determine the equation of the line through the points whose coordinates are $(2,-1)$ and $(5,-1)$.

Using the Slope Formula with $(x_1, y_1) = (2,-1)$ and $(x_2, y_2) = (5,-1)$

$\displaystyle m = \frac{-1-(-1)}{5-2} = \frac{0}{3} = 0
$

The slope of the line is $\displaystyle 0$.

Using the point slope formula with $\displaystyle m = 0$ and $(x_1, y_1) = (2,-1)$


$
\begin{equation}
\begin{aligned}

y - y_1 =& m(x - x_1)
&&
\\
y-(-1) =& 0(x-2)
&& \text{Substitute } m = 0, (x_1, y_1) = (2,-1)
\\
y + 1 =& 0
&& \text{Apply Distributive Property}
\\
y =& -1
&& \text{Write the slope-intercept form}

\end{aligned}
\end{equation}
$