\( ABCD \) is a parallelogram. Point \(P\) on \(AB\) divides it in the ratio \( AP : PB = 3 : 2 \), and point \(Q\) on \(CD\) divides it in the ratio \( CQ : QD = 7 : 3 \). Let \(R\) be the intersection of \( PQ \) and \(AC\). Then, \( AR : AC = a : b \), where \(a\) and \(b\) are positive coprime integers. What is \(a + b \)?