I was going through my engineering book and came across this free body diagram of a structural member. What I don't get is that how did they come up with the 6200N reaction force at the ends of the whole structure? Any help in understanding this would be great.

 

Oh and sorry for the blurry image. My phone is not that great at taking pictures at night =(

 

The photo is alright but the diagram itself seems to be deliberately unhelpful!

Presuming the two joins at the right of the diagram attach to a solid wall - call the top A and the bottom O. If you take moments of force based on O - the lower point in contact with the wall (point with 6200N to the left and 2400N to the right)

The beam exerts a force of 900N at 2 metres from pivot O
The weight at the end is 1500N and is 5 (3+2) metres from pivot O

 

What must the rotational force be at point O ?