http://i55.tinypic.com/21cb3vl.jpg

I need help with number 24.

Here is my attempt at trying to figure out the width of this triangle:

http://i54.tinypic.com/59slmt.png

And here is my attempt at trying to set this problem up:

http://i51.tinypic.com/35kiiwo.jpg

Please tell me if this is wrong or right. I cannot check my answer in the back of the book. Thank you for your time.