Vector Algebra

Let a, b be two vectors, and let P, Q and R be the points with position vectors a, b and a+ b, respectively, with respect to the origin O. If | a+ b|= 21, | a- b|=3, and a and ( a- b) are perpendicular to each other, then the area of the triangle OPR is

1. 3
2. 32
3. 3 32
4. 32