even if correct you can’t just write down a formula and use it without justifying what it does or where it comes from

Of course you can, if the formula is well-known and its applicability is obvious. To use a question a bit more at the appropriate level, if the question was "Justin was sitting on a bus travelling at 50 mph. He has been moving for 12 minutes, how far has he travelled?" you wouldn’t expect the student to write "the formula to calculate distance given speed and time is speed x time, therefore distance = 50 x 1/5 = 10 miles".

No, you’d just expect "distance = 50 x 1/5 = 10 miles". Or maybe at most "distance = speed x time = 50 x 1/5 = 10 miles", but even that is overkill if the student has demonstrated understanding via the former.