A particle just clears a wall of height $b$ at distance $a$ and strikes the ground at a distance $c$ from the point of projection. The angle of projection is:

The equation of the trajectory of a projectile is $y=x\tan \theta \left(1-\frac{x}{R}\right)$, where $R$ is the range.Here, the wall is at $x=a$ and has height $y=b$.The particle strikes the ground at distance $c$, so the range $R=c$.Substituting these values into the equation:
$$b=a\tan \theta \left(1-\frac{a}{c}\right)$$
$$b=a\tan \theta \left(\frac{c-a}{c}\right)$$
$$\tan \theta =\frac{bc}{a(c-a)}$$
$$\theta ={\tan }^{-1}\left(\frac{bc}{a(c-a)}\right)$$