Solving M_x=U_x |r X R|: Need Help?

[suspenc3]

Homework Statement

See attachment

Homework Equations

I was thinking : $$M_x=U_x |r X R|$$

where r is going to be the distance from any point on the x-axis to any point on the line of action of the force

The Attempt at a Solution

Im not really sure how to exactly do this problem...Ive been plugging in numbers hoping for the best but nothing good has come out of it...Any help would be appreciate.

 

[PhanthomJay]

Attachment, please, is missing.