public static double FindAngle(LineSegment fLine, LineSegment sLine)
{
var v1 = fLine.ConvertToVector();
var v2 = sLine.ConvertToVector();
double scalar = v1.X * v2.X + v1.Y * v2.Y;
double m1 = FindMagnitude(fLine);
double m2 = FindMagnitude(sLine);
return(Math.Acos(scalar / m1 * m2));
}