$解:∵∠MPN=90°,NQ⊥PQ,MS⊥PQ$
$∴∠PSM=∠Q=∠MPN=90°$
$∴∠SPM+∠PMS=90°,∠SPM+∠NPQ=90°$
$∴∠PMS=∠NPQ=90°-∠SPM$
$∴在△PMS和△NPQ中,$
$\begin{cases}{∠PMS=∠NPQ}\\{∠PSM=∠NQP}\\{PM=PN}\end{cases}$
$∴△PMS≌△NPQ(AAS)$
$∴MS=PQ,PS=NQ=2.1cm$
$∵QS=3.5cm$
$∴MS=PQ=QS+PS=2.1+3.5=5.6cm$