$\vec a . \vec b = 1.4+(-1).0 +1.(-1)= 3\Rightarrow \vec x = 3.(3.\ 2,\ -1)=(9,\,6,\ -3)$
$\vec c^2 = 9+4+1 = 14$
$\Rightarrow \vec y = 3(1,\ -1,\ 1) -6(4,\ 0,\ -1) =(35,\ -3,\ -5)$
$|\vec a|=\sqrt{26};\ |\vec b| =\sqrt{14}$
$\cos (\vec a,\ \vec b) =\dfrac{\vec a .\vec b}{| \vec a | .|\vec b|}=\dfrac{4.(-1)+3.2+1.3}{\sqrt{14.26}}=\dfrac{5}{2\sqrt{91}}$
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