7. In the diagram PR and QS are straight lines meeting at O.PQ is parallel to SR and the ratio \( P Q: S R=2: 3 O P=a \) and \( O S=b \).
(a)Express in terms of \( a \) and/or \( b \)
(i) \( O Q \)
(ii) RS
(b) The sum of the areas of triangles OPQ and \( ORS \) is \( 325 cm ^{2} \).calculate area of triangle ORS.
