First, on a technical note, can I suggest that when you've scanned or photographed a document, you save it as a .jpg file, and then attach it using the 'Manage Attachements' button. That way, it will be attached at the bottom of your message as a thumbnail. The problem with the attachment that you've made is that it is just too big!
Originally Posted by womatama
Now take a look at the diagram I've attached.
You'll see that I've marked all the angles that you'll need. (The lines AE and BC are vertical, and BF, CE and DE are horizontal, so we've got lots of right-angles I haven't marked, of course.)
Then you can find, in this order, the lengths:
BC (= EF), AF, BF (= CE), CD and hence DE.Can you do it now?