4.04: Parallel vectors in a parallelogram

\(OABC\) is a parallelogram with \(\overrightarrow{OA}=\mathbf a\) and \(\overrightarrow{OB}=\mathbf b\). \(D\) lies on \(OB\) with \(OD:OB=1:3\), and \(E\) lies on \(AC\) with \(AE:AC=2:3\).