[tex]\bf \begin{array}{lllll}
&x_1&y_1&x_2&y_2\\
% (a,b)
O&({{ 1}}\quad ,&{{ 2}})\quad
% (c,d)
N&({{ 3}}\quad ,&{{ 1}})
\end{array}
\\\\\\
% slope = m
slope = {{ m}}= \cfrac{rise}{run} \implies
\cfrac{{{ y_2}}-{{ y_1}}}{{{ x_2}}-{{ x_1}}}\implies \cfrac{1-2}{3-1}[/tex]
[tex]\bf y-{{ y_1}}={{ m}}(x-{{ x_1}})\qquad
\begin{array}{llll}
\textit{plug in the values for }
\begin{cases}
y_1=2\\
x_1=1\\
m=\boxed{?}
\end{cases}\\
\textit{and solve for "y"}
\end{array}\\
\left. \qquad \right. \uparrow\\
\textit{point-slope form}[/tex]
