Chapter 12.3, Problem 4QCE

Suppose that $\text{r}\left(t\right)$ is a smooth vector-valued function of t with $\text{r}\text{'}\left(1\right)=〈\sqrt{3},-\sqrt{3},-1〉,$ and let ${\text{r}}_1\left(t\right)$ be defined by the equation ${\text{r}}_1\left(t\right)=r\left(2\cos t\right).$ Then ${\text{r'}}_1\left(\pi /3\right)=\text{\_\_\_\_\_\_\_\_}\text{.}$