The problem is the use of the word "Minus".
The question should be;
If the compass heading is 180 degrees and the deviation is 5 degrees west, what is the magnetic heading?
We know all about CDMVT.
And every single aircraft i have operated i have had to go out to the runways and do my own compass swing.
I even had one aircraft returned to me from maintenance with compass 40 degrees off.
Supposedly new compass and swung...with no radios fitted!