Answer
See explanation
Work Step by Step
This is a simple matter of understanding the given marginal note. Notice that the line is parametrized by normalizing the given vector $\mathbf{v}$. At the point $(t_0)$, the unit tangent vector is: $$ \mathbf{T}(t_0) = \frac{\mathbf{v}}{\|\mathbf{v}\|} $$ So the rest of the task is a simple matter of renaming the variables correctly. From here, it follows that the line is parametrized with: $$ \mathbf{r} = \mathbf{r}(t_0) + s\mathbf{T}(t_0) $$
