Why Does the Solution Use FH(6) Instead of FH(10) for Gate Force Calculation?

[foo9008]

Homework Statement

the gate is 3m wide , is hinged at point A , and rests against a smooth wall at B . Calculate the force on the gate due to seawater pressure and horizontal force exerted by wall at B . I have attached the sample answer below . I don't understand why the author use FH(6 ) instead of FH(10)

Homework Equations

The Attempt at a Solution

IMO , it should be FH(10) , am i right ? 10 is the distance from A to B

 

[haruspex]

In what direction does F_H act?

 

[foo9008]

In what direction does F_H act?

Horizontal

 

[haruspex]

Horizontal

Right, so how do you find its moment about A?

 

[foo9008]

Ok, I forgot it's horizontal force