## Calculus (3rd Edition)

$$\sqrt{21}$$
We have $r'(t) =\lt 2,4,-1\gt$. Hence the speed $v(t)$ is given by $$v(t)=\|r'(t)\|=\sqrt{4+16+1}=\sqrt{21}.$$ At $t=4$, $v(4)=\sqrt{21}$