You have to glide. Get on that high pole that sticks out (the one right above the ?, with the tightrope attached) Jump out, make kindof a U-Turn midair, and snap the riddle on your way back in.
You can't actually land in the city below when you jump off Wonder Tower. I tried jumping off and gliding down to street level several times and it wouldn't let me. So if you fail, Batman will just grapple back up and you can try again.