Drag the points A and B and note that their positions are linked to the text on the right. Check the "Show Resultant" Checkbox to see how these two vectors would be added using the triangle law and show their resultant. Note that the point C now has it's coordinates displayed on the right as well. What relationship appears to exist between the coordinates of the points A, B and C? Move the points around to see if you are correct. Make sure you test this relationship with each point in all the different quadrants.
Remember, since these are all position vectors, the point at their tip describes the vector itself. What does this tell you about the addition of vectors? How would you go about calculating the sum of two vectors? Create a formula that explains this.