## Elementary Linear Algebra 7th Edition

$$c=-\pm1.$$
$$\| c(2,2,-1)\|=3\Longrightarrow |\|c (2,2,-1)\|^2=9 \Longrightarrow c^2 (4+4+1)=9$$ then, we get $c^2=1$ and hence $$c=-\pm1.$$