How does the force get cancel out?

[cmkc109]

Homework Statement

How does the force get cancel out?

Homework Equations

The Attempt at a Solution

Please see the picture.
I don't understand why is F1 not included in the system ?
F1 just looks the same as F4, but F4 is included in the calculation.
Someone told me the horizontal vector and vertical vector somehow cancel out each other but i don't quite get it..

 

[rock.freak667]

What exactly does the calculation involve?

 

[cmkc109]

solved the problem!