The problem can basically be restated by saying that you have three given vectors that define three sides of a quadrilateral, and you need to find a vector that defines the fourth side. This is a perfect example of when to use vector addition and subtraction. For convenience, place point O at the origin (0,0) and draw a vector, doesn't matter what vector. Stop at some point A, and draw another vector with point A as your base. Stop at a third point B. Note that you've drawn two sides of a triangle. So, how would you find the third side of the triangle, the vector OB? Now, can you extend this principle to a quadrilateral? As a hint, any quadrilateral can be divided into two triangles.
