#### Answer

undefined; vertical

#### Work Step by Step

We are given that the slope $m$ of the line passing through the distinct points $(x_{1},y_{1})$ and $(x_{2},y_{2})$ is defined as:
$m=\frac{rise}{run}=\frac{y_{2}-y_{1}}{x_{2}-x_{1}}$ (where $x_{2}\ne x_{1}$)
Therefore, $m=\frac{3-(-1)}{4-4}=\frac{3+1}{0}=\frac{4}{0}=undefined$.
The slope is undefined, so the line is vertical.