Answer
$\theta = 0.61~radians$
Work Step by Step
$\theta = tan^{-1}(\frac{4}{x})-tan^{-1}(\frac{1}{x})$
We can find $\theta$ when $x = 3$:
$\theta = tan^{-1}(\frac{4}{3})-tan^{-1}(\frac{1}{3})$
$\theta = 0.9273-0.3218$
$\theta = 0.61~radians$