The block will move if
$F \cos \theta \geq f_{\max }$
$F \cos \theta \geq \mu(W-F \sin \theta)$
$\mu=\tan \alpha=\frac{\sin \alpha}{\cos \alpha}$
$F \cos \theta \geq \frac{\sin \alpha}{\cos \alpha}(W-F \sin \theta)$
$F(\cos \theta \cos \alpha+\sin \theta \sin \alpha) \geq W \sin \alpha$
$F \geq \frac{W \sin \alpha}{\cos (\theta-\alpha)}$
$F_{\min }=\frac{W \sin \alpha}{\cos (\theta-\alpha)}$




