Using some code from msdn
http://msdn.microsoft.com/es-es/library/f956fzw1(v=vs.110).aspx[
^]
Starting point needs x y coords. Lets say you have them. And you have the angle in radians, and length:
int x = 500;
int y = 600;
decimal angle = 1.5;
int length = 400;
public void DrawLinePoint(PaintEventArgs e)
{
Pen blackPen = new Pen(Color.Black, 3);
Point point1 = new Point(x, y);
Point point2 = new Point(x + Math.Cos(angle)*length, y + Math.Sin(angle)*length);
e.Graphics.DrawLine(blackPen, point1, point2);
}