## Thomas' Calculus 13th Edition

$59.5^{ \circ}$
The the angle between vectors $v$ and $x$-axis is given as: $\theta = \cos ^{-1} (\dfrac{v_x}{|v|})$ Here, $v=\lt 3,-5,1 \gt$ $|v|=\sqrt{3^2+(-5)^2+(1)^2}= \sqrt {35}$ Thus, $\theta = \cos ^{-1} (\dfrac{3}{ \sqrt {35}})=59.5^{ \circ}$