[tex]\bf \begin{array}{lllll}
&x_1&y_1&x_2&y_2\\
% (a,b)
&({{ 4}}\quad ,&{{ 1}})\quad
% (c,d)
&({{ 4}}\quad ,&{{ 2}})
\end{array}
\\\\\\
% slope = m
slope = {{ m}}= \cfrac{rise}{run} \implies
\cfrac{{{ y_2}}-{{ y_1}}}{{{ x_2}}-{{ x_1}}}\implies \cfrac{2-1}{4-4}\implies \cfrac{1}{0}\leftarrow unde fined[/tex]
so... the slope is undefined, the graph of it is just a vertical line, check the picture below, and that's the equation.
now... you can't quite put it in y = mx + b, or slope-intercept form, since it has no defined slope and it doesn't have an y-intercept either.
