SOLUTION: If the bearing of A to B is 138°,then what is the bearing of B to A?

The other tutor just gave you the answer but didn't show you how
to get it.

Here is A and B:
             

Draw a line connecting them.

             

From A we draw a line of arbitrary length vertically upward (in green) 
to indicate the direction of north from A. 
             

That means that if we measure the angle indicated by the red arc below
IN THE CLOCKWISE direction, we find that this clockwise angle measures 
138°: 


             
Next we do a similar thing at B.  From B we draw a line of arbitrary length
vertically upward (also in green) to indicate the direction of north from B.



             

We are to find the number of degrees in the CLOCKWISE measured angle 
from A to B, indicated by the other red arc below:

             

What angle does that second red arc indicate the measure of?

To find out, one easy way is to extend the line from A to B at B, 
like this:

             

Then we can see that the angle indicated by the blue arc below is 
also 138°, because the two green lines are parallel and the 
transversal AB cuts them at B.  Then the rest of the angle indicated
by the red arc is 180°, so we add the two together: 

             

So the bearing from B to A is 138° + 180° or 318°.

Edwin