Can someone give me the solution in simple trig?
Two points constitute a line, but however if you are talking about two vectors from the origin, then we can find the angle between those VECTORS.
GLfloat findangle(vector one, vector two)
{
GLfloat dotproduct=one.xtwo.x + one.ytwo.y + one.ztwo.z;
GLfloat cosangle(dotproduct/(one.lengthtwo.length));
return acos(cosangle);
}