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