If you have a solid foundation then you are/should (be) able to find intersection of two lines at a point. Example: Slope (Trajectory/ angle of muzzle), starting point Altitude or position. Add this with the plane of the earth where you would like for the projectile to hit. Basically, y=mx+b, if you have any knowledge of Algebra you more than likely have seen this at some point of another, but on a 2D scale.