2018 BIMC keystage2-EMIC individual Question 5

In quadrilateral $ABCD$, $AB$ is parallel to $CD$, and the diagonals $AC$ and $BD$ are perpendicular and intersect at point $O$. If $AO=p$ cm, $CO=q$ cm, $BO=r$ cm, $DO=s$ cm, $AB=7$ cm and $CD=3$ cm, what is the value of $pq+rs$?