Answer
$tan^{-1}(\frac{L}{w})$
Work Step by Step
Recall, with an unstable equilibrium, the box is "on the edge," meaning that, if released, it will readily tip either direction. When considering the geometry behind this point, we can obtain the following expression:
$tan\theta = \frac{L}{w}$
$\theta = tan^{-1}(\frac{L}{w})$
