)to keep a door closed, a wooden stick is wedged between the floor and the doorknob. the stick exerts at b a 175 n force directed along line ab. replace that force with a equivalent force couple system at
c.

The 175 N force is in the path A to B since a stick can’t pull the doorknob, this can be written as: 

(F ) ⃗= 175⋅(73i ⃗+990 j ⃗+494 k ⃗)/(√73² +990²+ 494² )
(F ) ⃗=(11.5i ⃗+156  j ⃗+78.0k ⃗ )N  

The displacement form C to B is:
(r ⃗ )_(B/C)=(683 i ⃗+860  j ⃗+0k ⃗ )mm  

The formula for the moment about C is  
M ⃗_c= (r ⃗  )_(B/C)  x (F ) ⃗  
M ⃗_c= (683 i ⃗+860  j ⃗ )  x (683 i ⃗+860  j ⃗+0k ⃗ )mm.N 
M ⃗_c= (67.1 i ⃗+53.3  j ⃗+116 k ⃗ )m.N

Answer(b) both involve the use of levers;