This is the question:
Find the area of the rhombus PQRS if the coordinates of point P, Q and R are (6,4),(8,7),(-6,3) respectively.
Any one know how to solve this?
As that link shows, taking the magnitude of the cross product of two vectors also gives you the area of the parallelogram defined by those two vectors as the non-parallel sides. In your example, $Q$ is an end point of both vectors and as such we can find the vectors
Is that the answer you wanted?