Answer
$\theta =\tan ^{-1}\left( \dfrac {x}{2}\right) $
Work Step by Step
Lets apply law of sines to this right triangle
In a right triangle if one internal angle is $\theta $ other angle will be
$\alpha =180-90-\theta =90-\theta $ (sum of internal angles of triangle is 180)
$\dfrac {x}{\sin \theta }=\dfrac {2}{\sin \left( 90-\theta \right) }=\dfrac {2}{\cos \theta }\Rightarrow \tan \theta =\dfrac {x}{2}\Rightarrow \theta =\tan ^{-1}\left( \dfrac {x}{2}\right) $
