I was reviewing my lessons on our "Strength of Materials" and I came up with this problem.

I get stucked on this thing.

http://img197.imageshack.us/img197/9498/45268362.jpg

How did the final equation/formula came up?

I'm kinda curious.(Doh)(Crying)

help me please!!(Crying) can you explain it to me briefly? and as simple as possible..

thanks a bunch in advance! :D