A coefficient is the number attached to a term. In this case we want to know the number attached to the term c
14a - 72r - c - 34d
If there is no number visible in front of a coefficient then you can assume that there is an invisible 1 in front of it.
14a - 72r - 1c - 34d
This case the coefficient of the c-term is:
-1
The reason it is negative and not just positive 1 is because the subtraction sign acts as a negative sign
Hope this helped!
~Just a girl in love with Shawn Mendes