I believe this question should be a multiple choice question as there are infinite number of solutions.
Some of these solutions are (2 cards whose sum is -12): -12 and zero -11 and -1 -10 and -2 -9 and -3 -8 and -4 -7 and -5 -6 and -6 1 and -13 2 and -14 3 and -15 4 and -16 5 and -17 6 and -18 7 and -19 8 and -20 9 and -21 10 and -22
