Refer to my edited image of your branch.
Since the angle counterclockwise from A to B is 290 degrees, we can find the angle of BAC by subtracting 270 from it.
Once you have that angle BAC or CBA, you can solve the triangle using the sine/cosine rule. All you need to know is the length AC and BC. Once you have those lengths, you can add them to the co-ordinates of A(800,600) to get the co-ordinates of B.