12
The factor pairs of 18 are (18, 1) (9, 2) (6, 3)
Even considering their negative counterparts, none of them add up to -6.
-3 x -3 x 2 x -1 x -1 = 18
6x 3
63
113
120
60
29
15
63
-20
113
4005
18 and -5
-15
16 and 2
-137
120
60
18 and 10 18 x 10 = 180 18 + 10 = 28