Hi,
I heard contradictory information relating to the requirement concerning the “command word” calculate or justify with calculation:
Do we have to write down the detailed general formula before plugging the specific numbers in it according to the case?
You do not.
Indeed, you do not need to write anything but the correct answer to earn full marks.
If you fear that your answer may be incorrect, you can show your work, but, again, you needn’t write the formula with variables; simply write in the numbers. They know the formula.
Thank you Bill 
My pleasure.