Statics: Method of Joints (Truss)

[Vanessa Avila]

Can someone explain to me why that highlighted force is 0?

Homework Equations

∑Fx = 0

The Attempt at a Solution

I found F_DC to be 780lb and I am still trying to solve for F_DE

 

[PhanthomJay]

If BD was non zero, let's just say for example it was 100 pounds , then 100 lbs in the opposite direction would be required for equilibrium along that axis. Where would that required opposite force go?