empty document. We don't really like having to download docs either. Can you just type it in, or take a screen snapshot and crop it down and post the image?
Ok, It's all about the similar triangles.
Just note that ADE, ACB, EFB are all similar triangles and that
DE = CF, DC = EF
play around with the ratios given by the similar triangles and substitute in those equalities above and you'll get it.
Yell back if not.