∵∠BAC=90°
∴∠CAE+∠BAD=90°
∵BD⊥MN
∴∠BAD+∠ABD=90°
∴∠CAE=∠ABD
∵AB=AC,∠ADB=∠AEC=90°
∴△ACE≌△BAD
∴AD=CE=3,AE=BD=2
∴DE=AD+AE=3+2=5