Hello, I am new in this forum. I will start to be active on Javascript programming. Ive been struggling too hard on the following:

Im looking forward to do a simple app that allows me to draw lines and get the angle between the vectors drawed. (Mainly to compile it with Phonegap and use it on touch, when I touch a coordinate it will mark a point, and when I mark the point b it will draw the vector)
I have knowledge of the Math formula I need to apply in order to get the angle between specific vectors.
The problem Im having right now is that I dont know how to draw the arc between the 2 vectors and display the value close to it to show how many grades it has on the angle.
Ive seen some libraries like: http://jsdraw2dx.jsfiction.com/documentation.htm That are already coded to do vector drawing easily.
I also found many more methods to get the angle like for example: http://www.wikihow.com/Find-the-Angle-Be...wo-Vectors
What I want to do is this:


So I want to be able to mark any point in the graph (A) then mark B point to get a line automatically drawed and when I mark the C point it will draw the line between B and C and automatically get the Theta 90 grades in this case of the example.
Im struggling this because I do know how to get the grade, but dont know how to position it and draw the arc between the vectors.
Anyone knows how to reach a quick solution for this?

Please help, this is mainly to adapt and draw when I take picture, I can do that and use this as a second layer, but I cant draw the arc of the vector and display the angle right next to it.