No, it’s not the card game.
The puzzle is can simply be formulated: make 21 using the numbers 1, 5, 6 and 7 once each, by combining them with the operators + – * / and the ().
The word ‘numbers’ is on purpose, i do mean numbers and not digits. And there are no tricks involved such as turning digits around, making a 9 from the 6.
Kees Krol also brought this puzzle to my attention, again: Thank you, Kees!
You can check your solution