![\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]()
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.