The Angle between a Line and a Plane; an application of the Angle between two Lines 
Problem: 

A diagram of this is shown on the right.  v is the vector to which the line is parallel. P is the point of intersection of the line and plane. N is the normal to the plane through P. θ is the angle between the line and plane. α is the angle between the normal and the line. 

The angle, α, between the normal and the line can be easily found using 'the angle between two lines' method. The required angle, θ, is then the difference between α and one rightangle.
From the equation to the given plane, r.[3, 0, 4] = 5, the normal to the plane is parallel to the vector [3, 0, 4]. The line is parallel to the vector [2, 1, 0].

