Map Points calculate x and y for arrow

Dec 2019
Hello there.
I'm doing some software with Silverlight.

I'm drawing some Polylines in a Map, given some points.

In the first photo, there are shown three points.
They are connected with some esri polyline function.

But now I need to draw an arrow figure at the end of the line connected to the various points.
So for me to be able that arrow figure I need to figure the x and y point on which that arrow will be.
(For this I meant what can be seet on the second image x=? and y=?)

The system is coordinate system of the planet.

I will be very glad if you could help me.
The x and y data are not exactly accurate, but I hope you can tell me the way of calculating those points to simulate the arrow.

The angle of the arrow is important.


Jul 2008
Western Canada
Here's a little routine that I wrote in Basic a few months ago:
Sub DrawArrow(x1, y1, x2, y2)
  // Draws a line x1y1,x2y2 with arrowhead at the x2y2 end
  const deg2halfrad = 0.008726646259972
  const arrowLength = 20 'Pixels
  const arrowAngle = 30 'degrees
  // Calculate angle of line
  aLine = ATan2(y2-y1,x2-x1)
  // calculate angle of arrowhead lines
  ahLineA = aLine-mArrowAngle*deg2halfrad
  ahLineB = aLine+arrowAngle*deg2halfrad
  // calculate x and y coordinates of the arrowhead points
  ahLineAx = x2-mArrowLength*cos(ahLineA)
  ahLineBx = x2-mArrowLength*cos(ahLineB)
  ahLineAy = y2-mArrowLength*sin(ahLineA)
  ahLineBy = y2-mArrowLength*sin(ahLineB)
  // Draw the principal line
  // Draw two line segments for arrowhead
End Sub