If a projectile is fired from the origin(of the xy-plane) with an initial velocity v at an angle of theta above the horizontal, then its trajectory(ignoring air resistance) is the parabola

y = tan(theta) x - [g/(2v^2(cos(theta)^2)] * x^2 where 0<theta<Pi/2

where g is the gravitational constant.

Suppose the projectile is fired from the base of a plane that is inclined at a constant angle alpha,(alpha >0), upwards from the horizontal, and lands on this plane at the point P. The range R is the distance from the origin to P and clearly depends upon the initial angle theta. Show that

R(theta) = 2v^2/(g(cos alpha)^2) * cos(theta)sin(theta-alpha)

I have no clue as to how to start even. I really don't understand the problem. Could somebody draw up a diagram for me so that I can better understand the problem? I am not sure how alpha and theta are related. I don't know if I am having trouble understanding the English or the math. It's probably both!