#### Answer

$$\sqrt{21}$$

#### Work Step by Step

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 $t=4$, $v(4)=\sqrt{21}$

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Published by
W. H. Freeman

ISBN 10:
1464125260

ISBN 13:
978-1-46412-526-3

$$\sqrt{21}$$

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