Find the angle subtended by the line segment through (0, 0, 0) and (3, 8, 5) from the xy-plane.

From the figure below:
$\sin \theta = \dfrac{z}{d}$
 

 

$\sin \theta = \dfrac{5}{\sqrt{3^2 + 8^2 + 5^2}}$

$\theta = 30.34^\circ$
 

Or you can also do this:
$\tan \theta = \dfrac{5}{\sqrt{3^2 + 8^2}}$

$\theta = 30.34^\circ$