Ok this is how it goes:
A shell is fired with Velocity (V) with and angle of elevation (a[alpha]) from a gun at point (o).
From point (o) a hill extends at angle (theta)
Given:
x = V.t.cos(a)
y = -1/2 .g.t^2 + v.t.sin(a)
and the time when the shell collides with the ground is T = R.cos(theta) /...