Solution for marked question:

Applying equation of motion perpendicular to the incline for y=0

0=Vsin(θ-a)t+1/2*(-gcosα)t^{2}

⇒t=0&2Vsin(θ-α)/gcosα

At the moment of striking the plane, as velocity is perpendicular· to the inclined plane hence component of velocity along incline must be zero

0=vcos(θ-α)+(-gsinα)2Vsin(θ-α)/gcosα

vcos(θ-α)=tanα,2Vsin(θ-α)

cot(θ-α)=2tanα